document status of each branch; 1.5.1 status unclear

[lavalamp]

@caesarxuchao says the 1.5.x branch is not maintained and users should switch to the 2.0.x branch. I think we should at least clearly document the status of each branch. I also think we must give advance notice if a branch is deprecated, doubly so if we're no longer going to put bugfixes in it. As a consequence I don't think we can actually stop supporting the 1.5 branch at the moment. @caesarxuchao and I will figure out the best course of action here. I'm opening this issue so it's clear that we're aware of the issue and so there's a place for feedback about this.

[lavalamp]

We have blessed the 2.0.0 tag and documented the 1.5 branch status, and I added a deprecation policy so this doesn't surprise us again.