README.md: add instructions to create a gerrit account

A git cookie isn't enough, a user needs to associate their google
account with a gerrit one. Also clarify which users need to sign a CLA.

Fixes errors like:
remote: PERMISSION_DENIED: The caller does not have permission
remote: [type.googleapis.com/google.rpc.LocalizedMessage]
remote: locale: "en-US"
remote: message: "\'git push\' requires a Gerrit user account."
remote:
...
fatal: unable to access 'https://aomedia-review.googlesource.com/aom/':
The requested URL returned error: 403

Change-Id: I86ab89fcb5493b9300fdf5add9a2a12b577d8717

README.md

diff --git a/README.md b/README.md
index 12498f5..d7b66e0 100644
--- a/README.md
+++ b/README.md
@@ -574,12 +574,19 @@
 `Generate Password` Password link at the top of the page. You’ll be given
 instructions for creating a cookie to use with our Git repos.
 
+You must also have a Gerrit account associated with your Google account. To do
+this visit the [Gerrit review server](https://aomedia-review.googlesource.com)
+and click "Sign in" (top right).
+
 ### Contributor agreement {#contributor-agreement}
 
 You will be required to execute a
 [contributor agreement](http://aomedia.org/license) to ensure that the AOMedia
 Project has the right to distribute your changes.
 
+Note: If you are pushing changes on behalf of an Alliance for Open Media member
+organization this step is not necessary.
+
 ### Testing your code {#testing-your-code}
 
 The testing basics are covered in the [testing section](#testing-the-av1-codec)