Added documentation about using mercurial for contributing code.

docs/docs/howto_contribute.xml

@@ -28,10 +28,28 @@
          </ul>
 
         <!--   ****************************   CODE CONTRIBUTIONS  ****************************    -->
+        <h2>Contributing Code</h2>
 
-         Code contributions are also welcome, however, you should read over the coding guidelines below
+         Code contributions are also welcome.  I use <a href="http://mercurial.selenic.com/">Mercurial</a> 
+         for version control.  So the simplest
+         way to contribute code is to clone from the dlib repository, make your changes, and then email
+         them to me as a bundle file.  An example is shown below.
+
+<code_box>
+hg clone http://dclib.hg.sourceforge.net:8000/hgroot/dclib/dclib
+cd dclib
+[make changes to dlib source code]
+hg commit
+hg bundle dlibchanges.hg
+</code_box>
+Then <a href="mailto:davis@dlib.net">email me</a> the dlibchanges.hg file and I'll review it and get back to you.
+
+         <p> If you want to make a big change or feature addition, it's probably a good idea to talk to me about it first.  
+            Additionally, you should read over the coding guidelines below
             and try to follow them.  It is also probably a good idea to read the books Effective C++ and 
-         More Effective C++ by Scott Myers.   And as always, feel free to contact me if you have any questions.
+            More Effective C++ by Scott Myers.  Finally, if you are not familiar with Mercurial you also 
+            might want to read the excellent <a href="http://hginit.com">introduction</a> by Joel Spolsky.  
+         </p>
 
         
          <h2>Coding Guidelines</h2>