CONTRIBUTE.md: concentrate on the feature freeze and the roadmap
README.md: remove tarball and marmalade instructions

Using a tarball is no longer a supported installation method.  We
will continue to provide a tarball for distributions but users who
do not use their distributions native package manager, are strongly
recommended to use Elpa instead.

We will continue to make new releases available on Marmalade, and
once 2.1.0 has been released the README will once more explain how
to do so.
README.md: the other package is VC, not Version Control

From `(emacs)Version Control`:

  The Emacs version control interface is called "VC".