Clarify the need to log into Gerrit

Failing to do so causes errors such as:

remote: PERMISSION_DENIED: The caller does not have permission
remote: [type.googleapis.com/devtools.gerritcodereview.proto.GerritCodeReviewError]
remote: error_type: PERMISSION_DENIED_BY_GERRIT_ACL

Bug: documentation
Change-Id: Ic416ac218e4c50639a712ffcd8ef17fae3e876b8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/61788
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Sean Risser <srisser@google.com>
diff --git a/README.md b/README.md
index e6edd64..d8ce708 100644
--- a/README.md
+++ b/README.md
@@ -51,9 +51,9 @@
 https://swiftshader.googlesource.com/SwiftShader.

 

 All changes must be reviewed and approved in the [Gerrit](https://www.gerritcodereview.com/) review tool at:

-https://swiftshader-review.googlesource.com.

+https://swiftshader-review.googlesource.com. You must sign in to this site with a Google Account before changes can be uploaded.

 

-Authenticate your account here:

+Next, authenticate your account here:

 https://swiftshader-review.googlesource.com/new-password (use the same e-mail address as the one configured as the [Git commit author](https://git-scm.com/book/en/v2/Getting-Started-First-Time-Git-Setup#_your_identity)).

 

 All changes require a [Change-ID](https://gerrit-review.googlesource.com/Documentation/user-changeid.html) tag in the commit message. A [commit hook](https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks) may be used to add this tag automatically, and can be found at: