Added readme to tests directory.

tests/readme.rst

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+=================
+OpenMC Test Suite
+=================
+
+The purpose of this test suite is to ensure that OpenMC compiles using various
+combinations of compiler flags and options and that all user input options can
+be used successfully without breaking the code. The test suite is by no means
+complete and should not be viewed as a comprehensive unit test suite will full
+coverage. Until more effort can be put into actual unit testing, this suite is a
+simple means of making sure that new features added into the code don't break
+existing features.
+
+The test suite is designed to run with the third-party Python package
+nose_. Running the test suite is as simple as going to the tests/ directory and
+running:
+
+.. sh::
+    nosetests
+
+However, usually testing is split into two parts: compilation and running. To
+run the compilation tests, use:
+
+.. sh::
+    nosetests test_compile
+
+Then, to run all the normal tests (which require that an OpenMC executable is
+already built):
+
+.. sh::
+    nosetests --exclude-dir=test_compile
+
+.. _nose: https://nose.readthedocs.org