readme: Explain how to run tests

README.md

-xblock-utils
-============
+# xblock-utils: Various utilities for XBlocks
+
+These are a collection of useful utility functions,
+test base classes and documentation shared by many XBlocks.
+(Especially those of [edx-solutions](https://github.com/edx-solutions).)
+
+To test the utilities, run:
+
+    nosetests --with-coverage --cover-package xblockutils