Updates the contributing guide.

Resolves #1516

- Creates two new branches: major and minor. We will need to rebase major everytime an update to master or minor happens.
- Minor is a duplicate of v2.1 right now. Because we have 2.1 PRs open I didn't close that branch down. Instead we will need to start using minor once v2.1 lands and we begin work on v2.2.
- Updates the contributing guide to no longer reference the now-defunct dev branch.