Add link to Test Double bliki

I noticed you are already linking to the xUnitPatterns definitions and Martin Fowler's bliki Mocks Aren't Stubs.

I thought it would be good to link to Martin Fowler's Test Double bliki.  I feel it is a clear, compact summary of the information in the other two links.
Merge pull request #379 from maxlinc/patch-1

Add link to Test Double bliki