Add screenshots to the README

The README is written in Markdown which supports images. Having a couple
of screenshots in the README (and thus on the project page on Github)
would help new users immediately understand the value of git-gui.

The screenshots are chosen to illustrate some of the main features.

The images are scaled down and compressed with `optipng -v -o7 *.png` to
ensure they don't waste space despite being binary files.

Signed-off-by: Otto Kekäläinen <otto@kekalainen.net>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

Author: ottok

README.md

@@ -11,6 +11,13 @@ page](https://git-scm.com/docs/git-gui).
 Git GUI was initially written by Shawn O. Pearce, and is distributed with the
 standard Git installation.
 
+Examples of `git gui blame` and `git citool`, the most popular tools in the git-gui
+suite:
+
+![git gui blame example](docs/git-gui-blame-example.png)
+
+![git citool example](docs/git-citool-example.png)
+
 # Building and installing
 
 You need to have the following dependencies installed before you begin: