Add a "Contributing Patches" section to the HACKING file.
More comprehensive test for alpha blending with multisampling.

This test validates a corner case of the fix for
https://bugs.freedesktop.org/show_bug.cgi?id=53077 ([IVB] Output error
with msaa when both of framebuffer and source color's alpha are not
1).  Specifically, it verifies that the first time alpha blending is
used on a multisampled framebuffer, the contents of the framebuffer
are properly preserved.