Clarify the need to log into Gerrit
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 <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Sean Risser <2d04e98eeaa433fb55534399ab3ab2c06fdf7967@google.com>

google/swiftshader,google/swiftshader,google/swiftshader