tag versions on merging to master

[isiyu]

reminder ticket to tag versions when merging to master

TODO:

• update contributing docs to include versioning info, and semver http://semver.org/ link
• include in PR template reminders to update version when merging to master

@BergWerkGIS any feedback here?

cc @MateoV @david-rhodes

[wilhelmberg]

Sounds good to me.
I think it's ok how like we've been doing it already: