Add extension point to register JGit PostReceiveHooks

Plugins may want to get notified when a pack file is received.
For example, the quota plugin may want to cache the repository
size in the file system and update it every time a new pack is
received.

Change-Id: I0212eab6e4867819ebe7ecc7fc5532c26bb82282

Documentation/dev-plugins.txt

diff --git a/Documentation/dev-plugins.txt b/Documentation/dev-plugins.txt
index e0824ca..7593578 100644
--- a/Documentation/dev-plugins.txt
+++ b/Documentation/dev-plugins.txt
@@ -424,6 +424,14 @@
 plugins will make use of the setXXX methods on the ReceivePack to
 set additional properties on it.
 
+[[post-receive-hook]]
+== Post Receive-Pack Hooks
+
+Plugins may register PostReceiveHook instances in order to get
+notified when JGit successfully receives a pack. This may be useful
+for those plugins which would like to monitor changes in Git
+repositories.
+
 [[ssh]]
 == SSH Commands