Add documentation for contextUserId.

Change-Id: I1e1701a1a2495b486c04937e027337789ab3fb5f
diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 9410e04..d4b0a38 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -135,6 +135,13 @@
     missingDownstreamsMessage = Missing downstreams ${missingDownstreams}
   ```
 
+global.contextUserId
+: User ID to use for doing all automerger operations.
+
+  If set to a valid user ID, all automerger operations will be done using the
+  credentials of this user ID instead of the credentials of the user doing the
+  upstream operation.
+
 automerger.branch1:branch2.setProjects
 : Projects to automerge for.