Add documentation of versioning/branches to main README

[Zsailer]

Adds language around versioning.

Also bumps master to 0.3.0.dev.

[Zsailer]

Waiting on #162 to get merged.

[echarles]

What is the impact on the PR merge. E.g. a new feature is developed for next release 0.3.0:

1. Do we merge in both master and 0.3.0 branche(s)?
2. ...or do we we merge only in 0.3.0 and then before the release merge all in master?
3. ... or merge in master as now and create a 0.3.0 for the release?

I prefer option 3 for simplicity.

[rolweber]

I fully agree. Features should be developed for master. Release branches should get created with or after the release. When the release is properly tagged, branch creation can be delayed until there's a need for the branch.

[kevin-bates]

Thanks Zach - this will be very helpful. I just had a couple comments.

[Zsailer]

Thanks, all!

3. ... or merge in master as now and create a 0.3.0 for the release?

Yes, this is what I intended. I think @kevin-bates inline comment will help clarify this. Master represents the latest minor version. We won't create a branch for the latest version until we release that branch on PyPI. Once we make a release, we bump the version in master and add features there.

[Zsailer]

Ok, modified the language to reflect that we're only creating branches once a version is released. Let me know if you think this needs anymore editting.

[kevin-bates]

LGTM - thank you.