Document how to build/run unit tests

doc/unit-tests.txt

@@ -0,0 +1,33 @@
+Compiling/runing bitcoind unit tests
+------------------------------------
+
+bitcoind unit tests are in the src/test/ directory; they
+use the Boost::Test unit-testing framework.
+
+To compile and run the tests:
+cd src
+make -f makefile.unix test_bitcoin  # Replace makefile.unix if you're not on unix
+./test_bitcoin   # Runs the unit tests
+
+If all tests succeed the last line of output will be:
+*** No errors detected
+
+To add more tests, add BOOST_AUTO_TEST_CASE's to the existing
+.cpp files in the test/ directory or add new .cpp files that
+implement new BOOST_AUTO_TEST_SUITE's (the makefiles are
+set up to add test/*.cpp to test_bitcoin automatically).
+
+
+Compiling/running Bitcoin-Qt unit tests
+---------------------------------------
+
+Bitcoin-Qt unit tests are in the src/qt/test/ directory; they
+use the Qt unit-testing framework.
+
+To compile and run the tests:
+qmake bitcoin-qt.pro BITCOIN_QT_TEST=1
+make
+./bitcoin-qt_test
+
+To add more tests, add them to the src/qt/test/ directory,
+the src/qt/test/test_main.cpp file, and bitcoin-qt.pro.