Document how we do releases, how we build docker images, etc

[orangejulius]

We don't have much documentation to tell people how we manage releases for Pelias.

Some things that would be useful to mention:

• In general, we run a rolling release process where people are generally best off using the latest master branch.
• We publish releases using semantic-release and using semantic versioning
• Releases for each component of pelias are well described in the GitHub releases page for each repo
• Each release of the "primary" Pelias repositories (Pelias services, importers, schema) have Docker images built for each branch, and for each commit
• In the case where we have to make a breaking change, we will call this out in the release notes for the affected projects, and generally follow the rule that we will try to make breaking changes that are backwards/forwards compatible with at least 1 or 2 months leeway