README: say how to commit to the proposal repository

Currently contributors have to guess that the proposal repository
follows the Gerrit workflow and figure out on their own that this
means cloning the Gerrit repository rather than the GitHub repository.

State this explicitly.

Change-Id: Id2c2542635ad04837ed8d74efddd2160330ebfd2
Reviewed-on: https://go-review.googlesource.com/15310
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/README.md b/README.md
index 372618c..c713b0c 100644
--- a/README.md
+++ b/README.md
@@ -83,6 +83,8 @@
 	- The design doc should be checked in to [the proposal repository](https://github.com/golang/proposal/) as `design/NNNN-shortname.md`,
 	  where `NNNN` is the GitHub issue number and `shortname` is a short name
 	  (a few dash-separated words at most).
+	  Clone this repository with `git clone https://go.googlesource.com/proposal`
+	  and follow the usual [Gerrit workflow for Go](https://golang.org/doc/contribute.html#Code_review).
 	- The design doc should follow [the template](design/TEMPLATE.md).
 	- The design doc should address any specific issues asked for during the
 	  initial discussion.