doc: link to proposal process from contribution guidelines

Change-Id: I992cb1afeef498353d529238e508fa438d6c069c
Reviewed-on: https://go-review.googlesource.com/12912Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

doc/contribute.html

@@ -20,23 +20,24 @@ see <a href="gccgo_contribute.html">Contributing to gccgo</a>.)
 
 <p>
 The project welcomes submissions but please let everyone know what
-you're working on if you want it to become part of the main repository.
+you're working on if you want to change or add to the Go repositories.
 </p>
 
 <p>
-Before undertaking to write something new for the Go project, send
-mail to the <a href="https://groups.google.com/group/golang-nuts">mailing
-list</a> to discuss what you plan to do.  This gives everyone a
-chance to validate the design, helps prevent duplication of effort,
-and ensures that the idea fits inside the goals for the language
-and tools.  It also guarantees that the design is sound before code
-is written; the code review tool is not the place for high-level
-discussions.
+Before undertaking to write something new for the Go project, 
+please <a href="https://golang.org/issue/new">file an issue</a>
+(or claim an <a href="https://golang.org/issues">existing issue</a>).
+Significant changes must go through the
+<a href="https://golang.org/s/proposal-process">change proposal process</a>
+before they can be accepted.
 </p>
 
 <p>
-In short, send mail before you code.
-And don't start the discussion by mailing a change list!
+This process gives everyone a chance to validate the design,
+helps prevent duplication of effort,
+and ensures that the idea fits inside the goals for the language and tools.
+It also checks that the design is sound before code is written;
+the code review tool is not the place for high-level discussions.
 </p>
 
 <h2 id="Testing">Testing redux</h2>