Add hint to canonicalWebUrl in documentation

Change-Id: Ibcf4338acbff18d3105661734a74132f80cdf5f8

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index b7843a4..758c867 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -102,6 +102,10 @@
 Bugzilla plugin is able to automate status transition on the issues based on
 code-review actions performed on Gerrit; actions are performed on Bugzilla using
 the username/password provided during Gerrit init.
+
+When using this possibility, the plugin assumes that gerrit.canonicalWebUrl is set.
+If it is not, the plugin will throw exceptions.
+
 Transition automation is driven by `$GERRIT_SITE/issue-state-transition.config` file.
 
 Syntax of the status transition configuration file is the following: