Add contributing doc

[programmiri]

📜 Description

This PR adds a "CONTRIBUTING" file at our root level. We want to provide contributors a better understanding how they can work in our project.

💁 Detailed changes:

• adds "CONTRIBUTING.md"

• "Contributing Guidelines" are based on existing examples from the aiven repositories

• ❗️ I've made some slight changes to that:

  • I made some sentences more clear to read in order to make it more accessible (based on suggestions from Hemmingway.app)
  • I added a paragraph about the code of conduct and linked or COC. I feel it's important to make people aware of us following a COC, right in probably one of the first files a contributor would look at.

• I added a first draft for creating commit messages. I based the recommendations on the Conventional Commits and How to Write a Git Commit Message. The latter is the article that the aiven internal etiquette for commit messages is also based upon.

Feedback welcome!

I appreciate feedback for this first draft 🙏! The commit message guideline will be grow in the future. I tried to keep it simple for now and e.g. only add prefixes where I felt they will definitely be useful. This should be used as a proposal which we should align upon together.

Resolves: #94
Relates to: #99

[programmiri]

**❓ Question **

The linked article about commit messages also recommends:

• always start description with a capitalised letter
• never use a period at the end of a description

Should we add these also as recommendations (or other rules to form) or is that less important to us?

As you can see with my two commit messages in this PR, I'm already inconsistence because I've a hard time not using a . at the end 🙈

[jlprat]

Thanks for the PR!
I have some minor tweak-requests to the doc, but otherwise good content and tone

[jlprat]

LGTM