Add example to review command how to vote on project specific label

Change-Id: Ieac5a0b556dc040b823ba43c365fa2b06c22b163
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

Documentation/cmd-review.txt

diff --git a/Documentation/cmd-review.txt b/Documentation/cmd-review.txt
index ed31e3db3..df64124 100644
--- a/Documentation/cmd-review.txt
+++ b/Documentation/cmd-review.txt
@@ -105,6 +105,11 @@
 	$ ssh -p 29418 review.example.com gerrit review --verified +1 c0ff33
 =====
 
+Vote on the project specific label "mylabel":
+=====
+	$ ssh -p 29418 review.example.com gerrit review --label mylabel=+1 c0ff33
+=====
+
 Append the message "Build Successful". Notice two levels of quoting is
 required, one for the local shell, and another for the argument parser
 inside the Gerrit server: