Need to document release process

[scottswezey]

As something I only work on periodically, I seem to always forget somewhere I need to bump a version number, or some other minor step in the process of preparing a release. I should make a file with all of the steps documented for future-me to not stumble over this yet again.

Steps I know to include so far:

1. Update deps
2. Check Twilio docs for changes to available magic numbers.
3. Ensure tests pass, which confirms all magic numbers are correct and still in use at Twilio.
4. mix format & mix credo
5. Update changelog
6. Update readme freshness section with version number and current date
7. Update mix.exs version number.
8. git add and git commit.
9. Post a release on GitHub.
10. mix hex.publish (does code and docs)