Add a note about contribution priorities

It's been poorly documented so far that the current git-review
maintainers are avoiding addition of new features. Gerrit-based
change workflows are already confusing enough to new users, and
providing them with too many complex options risks making things
worse. This tool's goal is to do a few things well and simply in
support of an opinionated workflow, without becoming everything to
everyone.

Since git-review sees broad usage outside the OpenStack contributor
community, it is subject to a lot of drive-by feature additions. Add
a note to the contributing document indicating its maintainers
consider it to be mostly complete (modulo bug fixes, compatibility
improvements, adding test coverage), so that readers will consider
strongly whether their particular improvement is well suited to the
existing scope of the tool.

Change-Id: I11b9f898286b75177493af31e209c2853d78f245

CONTRIBUTING.rst

@@ -2,6 +2,16 @@
  Contributing to git-review
 ============================
 
+This tool is considered mostly feature-complete by its authors. It
+is meant to provide a simple, convenient tool for users of basic
+Gerrit change workflows. Contributions fixing bugs or regressions,
+maintaining support for newer Gerrit/Git releases and improving test
+coverage are welcome and encouraged. It is not, however, intended as
+an all-encompassing Gerrit client (there are plenty of other tools
+available supporting more advanced interactions), so proposed
+feature additions may make more sense implemented as complimentary
+``git`` subcommands or similar related but separate projects.
+
 To get the latest code, see: https://git.openstack.org/cgit/openstack-infra/git-review
 
 Bugs are handled at: https://storyboard.openstack.org/#!/project/719