docs: Add CONTRIBUTING.md

[orpheuslummis]

Relevant issue(s)

Resolves #29

Description

The goal of the CONTRIBUTING.md is to be guidance to new and existing contributors to the repository.

I excluded or haven't a few things as they aren't ready or can be in a different document:

• Signing release
• Debugging recommendations
• Cross-compiling
• stuff about digital signatures
• stuff about using graphql clients
• relating to public versus internal
• Performance analysis
• nightly runs
• security-related policies
• community guidelines

Starting as a draft PR. You're very welcome to give feedback and general commentary.

Tasks

• I made sure the code is well commented, particularly hard-to-understand areas.
• I made sure the repository-held documentation is changed accordingly.
• I made sure the pull request title adheres to the conventional commit style (the subset used in the project can be found in tools/configs/chglog/config.yml).
• I made sure to discuss its limitations such as threats to validity, vulnerability to mistake and misuse, robustness to invalidation of assumptions, resource requirements, ...

How has this been tested?

Reviews from human contributors.

[orpheuslummis]

This comment/post maintains a link to preview the latest version of the markdown file for your convenience.

https://github.com/sourcenetwork/defradb/blob/d0c2722679391b7aba63e1087854bb33e78f8636/CONTRIBUTING.md

[orpheuslummis]

This may enable https://github.com/sourcenetwork/defradb/contribute
(see https://github.com/ethereum/go-ethereum/contribute for a working example)

[jsimnz]

love it! LGTM

[orpheuslummis]

love it! LGTM

Thanks. I suggest for this PR to wait for few days more for the core team's feedback and Pavneet's feedback, then getting it merged.

[shahzadlone]

Seems in harmony with the discussions we had =). Giving an approval even though it's draft right now haha.

[fredcarle]

Thanks for actioning my suggestions. 👍

[orpheuslummis]

Pavneet will contribute feedback later!