[PATCH] add to gitk documentation

A very brief couple of sentences on the basic "click" usage of gitk
which I can't find explicitly documented anywhere else.

Signed-off-by: David Tweed <redacted>
-- 
 Documentation/gitk.txt |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/Documentation/gitk.txt b/Documentation/gitk.txt
index e9f82b9..d9fb0a7 100644
--- a/Documentation/gitk.txt
+++ b/Documentation/gitk.txt

@@ -15,6 +15,13 @@ Displays changes in a repository or a selected set

of commits. This includes
 visualizing the commit graph, showing information related to each commit, and
 the files in the trees of each revision.

+The basic idea in gitk is that clicking on an element related to a commit
+(either the nodes in the commit graph or the commit brief message)
+provides both metadata and either a diff with the immediate parent or
+the tree contents of the commit (depending whether the patch or tree view
+is chosen). Clicking on a yellow tag marker provides the object id of the
+commit it references.
+
 Historically, gitk was the first repository browser. It's written in tcl/tk
 and started off in a separate repository but was later merged into the main
 git repository.