Update GN documentation.

Change-Id: I1827b9511a830240b2c14eb594ff48be761a0365
Reviewed-on: https://gn-review.googlesource.com/2681
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>

README.md

diff --git a/README.md b/README.md
index 0f30089..5c98d1f 100644
--- a/README.md
+++ b/README.md
@@ -23,18 +23,20 @@
 GN uses [Gerrit](https://www.gerritcodereview.com/) for code review. The short
 version of how to patch is:
 
+    Register at https://gn-review.googlesource.com.
+
     ... edit code ...
     ninja -C out && out/gn_unittests
 
 Then, to upload a change for review:
 
     git commit
-    git push origin HEAD:refs/for/master  # This uploads for review.
+    git cl upload --gerrit
 
 When revising a change, use:
 
     git commit --amend
-    git push origin HEAD:refs/for/master
+    git cl upload --gerrit
 
 which will add the new changes to the existing code review, rather than creating
 a new one.