merge-partner-on-aosp: Document usage

Issue: INFRA-184
Change-Id: Ifa6e49ce4a5ad3994022311391a3b858419665d5
diff --git a/bin/merge-partner-on-aosp b/bin/merge-partner-on-aosp
index 3724dab..97aaa57 100755
--- a/bin/merge-partner-on-aosp
+++ b/bin/merge-partner-on-aosp
@@ -15,6 +15,30 @@
 # limitations under the License.
 #
 
+# Merge partner security branches on AOSP projects
+#
+# Use this script together with the repo tool in the root of the Android
+# tree to merge partner security branches in supported projects.
+#
+# Following prerequisites need to be met:
+#   * Run `fetch-and-push-partner` first to have up-to-date copies of
+#     the security branches in the (internal) projects that are
+#     referenced in your Android manifest.
+#   * Make sure to `repo init` and `repo sync` with a manifest that
+#     points to project states you want to merge on. The manifest must
+#     point to branches, not tags or fixed commits; otherwise merges
+#     cannot be pushed (as there would be no branch to push to).
+#
+# Finally, use the following command line as reference to use the tool.
+# Set REVIEW_TOPIC and AOSP_PARTNER_SPL accordingly and run first
+# without PUSH_TO_REMOTE, to check the merge locally before pushing.
+#
+# repo forall -g aosp -vpc \
+#     "PUSH_TO_REMOTE=1 REVIEW_TOPIC=... AOSP_PARTNER_SPL=... \
+#     "$(pwd)/vendor/fairphone/tools/bin/merge-partner-on-aosp" 2>&1 | \
+#     tee merge-partner-on-aosp-$(date +"%Y-%m-%d_%H-%M-%S").log
+#
+
 # There are only a limited amount of valid situations:
 # 1- The project is not pointing to AOSP, ignore it;
 # 2- The AOSP-Partner reference did not move from the AOSP reference;