Update documentation about mandatory properties

Gerrit instance id is a mandatory setting for multi-site. Without that
property Gerrit node with multi-site will not start. Make sure that
documenation includes that information.

Bug: Issue 14660
Change-Id: Ifaca883a8d00fca6bc0ef8f7438b64578a3f8686

src/main/resources/Documentation/about.md

diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
index 623871b..159e4a0 100644
--- a/src/main/resources/Documentation/about.md
+++ b/src/main/resources/Documentation/about.md
@@ -7,7 +7,8 @@
 message broker for aligning with the other masters over different sites.
 
 The masters must be:
-
+* Gerrit instance id is mandatory for @PLUGIN@ plugin. All the master 
+  must have [gerrit.instanceId](https://gerrit-review.googlesource.com/Documentation/config-gerrit.html#gerrit) populated.
 * events-broker library must be installed as a library module in the
   `$GERRIT_SITE/lib` directory of all the masters
 * global-refdb library must be installed as a library module in the