Readme updates

README.md

@@ -125,12 +125,18 @@ To run all tests, run the script `runtests.py` in the mypy repository:
 
 Note that some tests will be disabled for older python versions.
 
+This will run all tests, including integration and regression tests,
+and will type check mypy and verify that all stubs are valid. You can also
+run unit tests only, which run pretty quickly:
+
+    $ ./runtests.py unit-test
+
 You can run a subset of test suites by passing postive or negative filters:
 
     $ ./runtests.py lex parse -x lint -x stub
 
-If you want to run individual unit tests, you can run myunit directly, or
-pass inferior arguments via -a:
+If you want to run individual unit tests, you can run `myunit` directly, or
+pass inferior arguments via `-a`:
 
     $ scripts/myunit -m mypy.test.testlex -v '*backslash*'
     $ ./runtests.py mypy.test.testlex -a -v -a '*backslash*'