doc: Update Unit Test Readme

src/test/README.md

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+The sources in this directory are unit test cases. The unit tests uses Boost's unit testing framework. 
+Since Gridcoin already uses Boost, Gridcoin uses the framework instead of requiring developers
+to configure another framework (reduces barriers to creating unit tests).
+
+The build system is setup to compile an executable called "test_gridcoin"
+that runs all of the unit tests. The main source file is called
+test_gridcoin.cpp, which simply includes other files that contain the
+actual unit tests (outside of a couple required preprocessor
+directives). The pattern is to create one test file for each class or
+source file for which you want to create unit tests. The file naming
+convention is "<source_filename>_tests.cpp" and such files should wrap
+their tests in a test suite called "<source_filename>_tests". For an
+examples of this pattern, examine uint160_tests.cpp and uint256_tests.cpp.
+
+The tests in transaction_tests.cpp are edge cases of Gridcoin transactions.
+They are in their current state not relevant for Gridcoin. Unusual transactions
+should be collected again from the gridcoin blockchain and replace
+the current test cases.
+
+For further reading, [see Boost's documentation](https://www.boost.org/doc/libs/1_73_0/libs/test/doc/html/boost_test/intro.html)
+about how the boost unit test framework works