doc: update CONTRIBUTING.md a bit, mention proposal process

Fixes #18550

Change-Id: Ia08d0ef6964216fcc14fa63c2ba378d68daa2c02
Reviewed-on: https://go-review.googlesource.com/34917Reviewed-by: Chris Broadfoot <cbro@golang.org>

CONTRIBUTING.md

@@ -7,6 +7,11 @@ It is the work of hundreds of contributors. We appreciate your help!
 
 ## Filing issues
 
+General questions should go to the
+[golang-nuts mailing list](https://groups.google.com/group/golang-nuts) or
+[other forum](https://golang.org/wiki/Questions) instead of the issue tracker.
+The gophers there will answer or ask you to file an issue if you've tripped over a bug.
+
 When filing an issue, make sure to answer these five questions:
 
 1. What version of Go are you using (`go version`)?
@@ -15,8 +20,7 @@ When filing an issue, make sure to answer these five questions:
 4. What did you expect to see?
 5. What did you see instead?
 
-General questions should go to the [golang-nuts mailing list](https://groups.google.com/group/golang-nuts) instead of the issue tracker.
-The gophers there will answer or ask you to file an issue if you've tripped over a bug.
+For change proposals, see [Proposing Changes To Go](https://github.com/golang/proposal/).
 
 Sensitive security-related issues should be reported to [security@golang.org](mailto:security@golang.org).
 
@@ -28,6 +32,7 @@ before sending patches.
 **We do not accept GitHub pull requests**
 (we use [an instance](https://go-review.googlesource.com/) of the
 [Gerrit](https://www.gerritcodereview.com/) code review system instead).
+Also, please do not post patches on the issue tracker.
 
 Unless otherwise noted, the Go source files are distributed under
 the BSD-style license found in the LICENSE file.