docs: add link back github repository

When people stumble upon this documentation at e.g.
 http://prototype-kernel.readthedocs.io/en/latest/prototype-kernel/index.html

Then it might not be obvious what GitHub repository this describes.
Thus, add link back to GitHub repo:
 https://github.com/netoptimizer/prototype-kernel

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

kernel/Documentation/prototype-kernel/index.rst

@@ -2,12 +2,14 @@
 Prototype Kernel
 ================
 
-This documentation is for how to use the prototype kernel project itself.
+This documentation is for how to use the `prototype-kernel project`_ itself.
+
+.. _`prototype-kernel project`: https://github.com/netoptimizer/prototype-kernel
 
 Prototype Kernel own documentation
 ==================================
 
-The prototype-kernel project is meant for compiling kernel modules
+The `prototype-kernel project`_ is meant for compiling kernel modules
 outside the normal kernel git tree, but still using the kernels make
 system.