Add documentation to help with debugging hook problems

A common problem for users of the plugin is that they are not sure
if their hooks are actually being executed, and what their output

Such information is logged by the plugin, but only at debug level.

Add a section in the documentation showing how to enable the logging.

Change-Id: I6e040f958ce4503f5cc0468ebe09466914d4a7eb
diff --git a/src/main/resources/Documentation/ b/src/main/resources/Documentation/
index 949b3b0..8e845f9 100644
--- a/src/main/resources/Documentation/
+++ b/src/main/resources/Documentation/
@@ -90,4 +90,20 @@
 of an SSH context (for example the patchset-created hook) don't know
 the server's web URL, unless this variable is configured.
+Debugging Hooks
+After execution of a hook, its exit code and any output are logged at
+debug level. To make this visible in Gerrit's log file, debug logging
+must be enabled for `com.googlesource.gerrit.plugins.hooks.HookTask`.
+This can be done by editing the `` file (requires a
+Gerrit restart) or by setting the log level at runtime with the ssh
+  ssh -p 29418 user@gerrit gerrit logging set-level DEBUG com.googlesource.gerrit.plugins.hooks.HookTask
 [1]: ../../../Documentation/config-gerrit.html#gerrit.canonicalWebUrl