Clarification of unit test build instructions.

src/test/README.md

@@ -16,6 +16,8 @@ their tests in a test suite called "<source_filename>_tests".  For an
 examples of this pattern, examine uint160_tests.cpp and
 uint256_tests.cpp.
 
+Add the source files to /src/Makefile.test.include to add them to the build.
+
 For further reading, I found the following website to be helpful in
 explaining how the boost unit test framework works:
 [http://www.alittlemadness.com/2009/03/31/c-unit-testing-with-boosttest/](http://www.alittlemadness.com/2009/03/31/c-unit-testing-with-boosttest/).