docs: update contributing guidelines (#10223)

## Description

Closes: #9814

+ Updates the contributing page
+ Adds contributing guidelines 

NOTE:
+ This pr doesn't update the code owners section. We will still need to define it.

This is based on the discussion we had 2 months ago and recent chats.

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [x] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [x] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

CODING_GUIDELINES.md

@@ -0,0 +1,87 @@
+# Coding Guidelines
+
+This document is an extension to [CONTRIBUTING](./CONTRIBUTING.md) and provides more details about the coding guidelines and requirements.
+
+## API & Design
+
++ Code must be well structured:
+  + packages must have a limited responsibility (different concerns can go to different packages),
+  + types must be easy to compose,
+  + think about maintainbility and testability.
++ "Depend upon abstractions, [not] concretions".
++ Try to limit the number of methods you are exposing. It's easier to expose something later than to hide it.
++ Take advantage of `internal` package concept.
++ Follow agreed-upon design patterns and naming conventions.
++ publicly-exposed functions are named logically, have forward-thinking arguments and return types.
++ Avoid global variables and global configurators.
++ Favor composable and extensible designs.
++ Minimize code duplication.
++ Limit third-party dependencies.
+
+Performance:
++ Avoid unnecessary operations or memory allocations.
+
+Security:
++ Pay proper attention to exploits involving:
+  + gas usage
+  + transaction verification and signatures
+  + malleability
+  + code must be always deterministic
++ Thread safety. If some functionality is not thread-safe, or uses something that is not thread-safe, then clearly indicate the risk on each level.
+
+
+## Testing
+
+Make sure your code is well tested:
++ Provide unit tests for every unit of your code if possible. Unit tests are expected to comprise 70%-80% of your tests.
++ Describe the test scenarios you are implementing for integration tests.
++ Create integration tests for queries and msgs.
++ Use both test cases and property / fuzzy testing. We use the [rapid](pgregory.net/rapid) Go library for property-based and fuzzy testing.
++ Do not decrease code test coverage. Explain in a PR if test coverage is decreased.
+
+We expect tests to use `require` or `assert` rather than `t.Skip` or `t.Fail`,
+unless there is a reason to do otherwise.
+When testing a function under a variety of different inputs, we prefer to use
+[table driven tests](https://github.com/golang/go/wiki/TableDrivenTests).
+Table driven test error messages should follow the following format
+`<desc>, tc #<index>, i #<index>`.
+`<desc>` is an optional short description of whats failing, `tc` is the
+index within the test case table that is failing, and `i` is when there
+is a loop, exactly which iteration of the loop failed.
+The idea is you should be able to see the
+error message and figure out exactly what failed.
+Here is an example check:
+
+```go
+<some table>
+for tcIndex, tc := range cases {
+  <some code>
+  resp, err := doSomething()
+  require.NoError(err)
+  require.Equal(t, tc.expected, resp, "should correctly perform X")
+```
+
+## Quality Assurance
+
+We are forming a QA team that will support the core Cosmos SDK team and collaborators by:
+- Improving the Cosmos SDK QA Processes
+- Improving automation in QA and testing
+- Defining high-quality metrics
+- Maintaining and improving testing frameworks (unit tests, integration tests, and functional tests)
+- Defining test scenarios.
+- Verifying user experience and defining a high quality.
+    - We want to have **acceptance tests**! Document and list acceptance lists that are implemented and identify acceptance tests that are still missing.
+    - Acceptance tests should be specified in `acceptance-tests` directory as Markdown files.
+- Supporting other teams with testing frameworks, automation, and User Experience testing.
+- Testing chain upgrades for every new breaking change.
+    - Defining automated tests that assure data integrity after an update.
+
+Desired outcomes:
+
+- QA team works with Development Team.
+- QA is happening in parallel with Core Cosmos SDK development.
+- Releases are more predictable.
+- QA reports. Goal is to guide with new tasks and be one of the QA measures.
+
+
+As a developer, you must help the QA team by providing instructions for User Experience (UX) and functional testing.