Added all active features to the list in reference.md.
Added all active features to the list in reference.md.

Added a second note about keeping the reference.md list up-to-date to
the bottom of the list, since not everyone (including me) reads the
big comment at the top of it.  :)

Ensured that the feature gate list in reference.md is kept in
alphabetical order.

rust-lang/reference,rust-lang/reference