Add 'how to run this one test' note to test quick start. (#3436)

This was something I was looking for in the quick start, and didn't find
clearly stated anywhere in the README.

Author: carljm

test-data/unit/README.md

@@ -41,6 +41,7 @@ usually, "E: " is preferred because it makes it easier to associate the
 errors with the code generating them at a glance, and to change the code of
 the test without having to change line numbers in `[out]`
 - an empty `[out]` section has no effect
+- to run just this test, use `pytest -k testNewSyntaxBasics -n0`
 
 
 Fixtures