Add CONTRIBUTING.md

- This makes it easy for someone new to start working on BotKube
- Adds instructions to build BotKube from source code

Signed-off-by: Bhavin Gandhi <bhavin7392@gmail.com>

CONTRIBUTING.md

@@ -0,0 +1,56 @@
+# How to Contribute to BotKube
+
+We'd love your help!
+
+BotKube is [MIT Licensed](LICENSE) and accepts contributions via GitHub
+pull requests. This document outlines some of the conventions on development
+workflow, commit message formatting, contact points and other resources to make
+it easier to get your contribution accepted.
+
+We gratefully welcome improvements to documentation as well as to code.
+
+## Compile BotKube from source code
+Make sure you have `go` compiler installed. BotKube uses
+[`dep`](https://github.com/golang/dep) to manage dependencies. You
+will also need `make`
+and [`docker`](https://docs.docker.com/install/) installed on your machine.
+1. Clone the source code
+   ```sh
+   $ git clone https://github.com/infracloudio/botkube.git $GOPATH/src/github.com/infracloudio/botkube
+   $ cd $GOPATH/src/github.com/infracloudio/botkube
+   ```
+2. Build the container image
+   ```sh
+   $ make build
+   ```
+   This will build BotKube and create a new container image tagged as `infracloud/botkube:latest`
+
+3. Build BotKube binary (optional)  
+   If you don't want to build the container image, you can build the
+   binary like this,
+   ```sh
+   # Fetch the dependencies
+   $ dep ensure
+   # Build the binary
+   $ go build ./cmd/botkube/
+   ```
+## Making A Change
+
+*Before making any significant changes, please [open an
+issue](https://github.com/infracloudio/botkube/issues).* Discussing
+your proposed changes ahead of time will make the contribution process
+smooth for everyone.
+
+Once we've discussed your changes and you've got your code ready, make
+sure that builds mentioned in 1 and 2 pass and open your pull request
+against `develop` branch.
+
+Make sure your pull request has a [good commit
+message](https://chris.beams.io/posts/git-commit/):
+* Separate subject from body with a blank line
+* Limit the subject line to 50 characters
+* Capitalize the subject line
+* Do not end the subject line with a period
+* Use the imperative mood in the subject line
+* Wrap the body at 72 characters
+* Use the body to explain _what_ and _why_ instead of _how_