Replace contribution list in CONTRIBUTE file with link

Replace contribution list in CONTRIBUTE file with link

The GDB wiki page has a much better contribution checklist than
that in the GDB CONTRIBUTE file.  In addition, the wiki is easier
to keep up to date with current processes.

Reduce the CONTRIBUTE file down to a short paragraph followed by
a link to the contribution process.  This also ensures anyone
reading the CONTRIBUTE file for a given release has access to the
latest processes.

gdb/ChangeLog:

	* CONTRIBUTE: Replace contribution list with wiki link.