Update SUBMITTING_PATCHES to point to Contributor Guide

The process is likely to change as we experiment with Gerrit Code
Review and learn the Eclipse IP process.  Lets point the potential
contributor at a wiki that we can more easily keep up-to-date.

Change-Id: I99f7cb9edadd0f23024f59ec139dd1e35f5fbd3f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>