Add '-211', update '-210' and fix README

This pull request is primarily thought to get clarifications from @paulp (based on your answers, these proposed changes may be totally invalid. I hope it won't bother or hurt too much): 

After reading/fixing README.md, I'm not sure what is the "current" purpose of options like `-210`, `-29` (it seems like "long time ago" it was used to refer to the SNAPSHOT versions). I see like three possible variants:

a) latest **final** releases (e.g. 2.10.2). In this case, the addition of `-211` is proposed too early, but `-210` is already set to 2.10.3, which is not yet released.

or

b) latest release including release candidates and other tagged milestones (e.g. 2.11.0-M4  or 2.10.3-RC1)

or

c) snapshots (e.g. 2.11.0-SNAPSHOT)


In this PR and also in #58, I assumed that you removed on the long term the support of `-sbt-snapshot` option (but f19a3e4fe1dab69c06e249a2db3cc1934b7bf4b9 sounds maybe like a temporary workaround). Do you plan to re-introduce this option later? I wonder if it has been removed by accident while removing support of "scala snapshots" (certainly in relation with sbt/sbt#856). BTW, I noticed there was a `http://`duplicate typo in [url="http://http://repo.typesafe.com..."](https://github.com/paulp/sbt-extras/commit/f19a3e4fe1dab69c06e249a2db3cc1934b7bf4b9#L0L204).

Many thanks in advance for your clarifications!