Add unstable documentation.

Add unstable documentation.

I kept most of these relatively brief with the intent that more complete
documentation would be added when a feature is stabilized.  I did my best to be
accurate, but I am unfamiliar with most of these features, so please let me know
if anything should change.

@matklad: I didn't fully understand the use case for `minimal-versions`, so I
didn't say much about it.  In particular, I didn't understand why one wouldn't
just use `~` or `=` semver requirements if you wanted to be careful about not
pulling in a newer version.  When someone says "1.0", aren't they explicitly
saying they want a newer version (up to 2.0) if it's available?  The example
in the RFC of switching a dependency to "=1.0" sounds like a breaking change
(essentially downgrading a dependency).