Add note about Gerrit account creation

Description:
Creating just a Gerrit password isn't enough.  Before you can push a
change to Gerrit, you must also create/associate a Gerrit account with
the google account used to create the password.

This avoids "git push ..." rejections like this:

  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."

Change-Id: Id02c1a69ccb0c2b8bf4c63b77ed3064125966eb3
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/50985
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

CONTRIBUTING.md

@@ -22,7 +22,10 @@ use [Gerrit](https://boringssl-review.googlesource.com) for this purpose.
 If you have not done so on this machine, you will need to set up a password for
 Gerrit. Sign in with a Google account, visit
 [this link](https://boringssl.googlesource.com/), and click the "Generate
-Password" link in the top right. You will also need to prepare your checkout to
+Password" link in the top right. You must also have a Gerrit account associated with
+your Google account.  To do this visit the [Gerrit review server](https://boringssl-review.googlesource.com)
+and click "Sign in" (top right).
+You will also need to prepare your checkout to
 [add Change-Ids](https://gerrit-review.googlesource.com/Documentation/cmd-hook-commit-msg.html)
 on commit. Run: