Developer documentation

[orangejulius]

After all these years, we surprisingly don't have much, if any, developer specific documentation! 🤦‍♀️

While related to #263, this would be documentation specifically intended to get people up to speed on contributing to Pelias.

It would include things like:

• how to submit pull requests
• how we use Semantic Release and commit messages to manage versions
• how contributors specifically can (and should) push branches directly to our repos, and how those branches will have docker images created for them
• which testing libraries we use, how we prefer tests are written or updated, etc

[orangejulius]

I've been working a bit on this and will be opening a PR soon. If anyone has suggestions about things to include in addition to what I've listed above, let me know!