Clarify contribution and release guidelines

[zkamvar]

Currently, it's not exactly clear how the deployer is to be updated/maintained/released. Some things that would be good to document in a contributing guidelines document:

• How to add new packages to the repository
• Release procedure to avoid hitting github's filesize limit: https://help.github.com/en/articles/distributing-large-binaries
• Release schedule
• Checklist for things to update in the README

Some of these things can be automated (e.g. the release procedure), but it's good to make sure these are all up-to-date and clear so that I can die a gruesome death and we can continue making deployers without skipping a beat.