Include usage instructions in update-marl.sh

Since we had some trouble figuring out the intended usage of this
script, add instructions in the comments and make it
clear that force-submitting is required.

Change-Id: I8ddbfe367d2545914761898f09592485b3a72c38
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/74088
Reviewed-by: Shahbaz Youssefi <syoussefi@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Shahbaz Youssefi <syoussefi@google.com>

third_party/update-marl.sh

diff --git a/third_party/update-marl.sh b/third_party/update-marl.sh
index 09aa245..03d9e94 100755
--- a/third_party/update-marl.sh
+++ b/third_party/update-marl.sh
@@ -2,7 +2,15 @@
 
 # update-marl merges the latest changes from the github.com/google/marl into
 # third_party/marl. This script copies the change descriptions from the squash
-# change into the top merge change, along with a standardized description.
+# change into the top merge change, along with a standardized description. It
+# should be run from the top of the swiftshader directory, followed by
+# `git cl upload`.
+#
+# IMPORTANT NOTE: that git subtree doesn't play nicely with gerrit, so this script will
+# result in two separate CLs, the first of which will have a merge conflict.
+# You should force-submit the second (child) CL, ignoring the merge conflict on
+# the parent. Make sure you're up-to-date beforehand, to avoid the possibility
+# of a real merge conflict!
 REASON=$1
 
 if [ ! -z "$REASON" ]; then