Document show_contexts (#473)

docs/contexts.rst

@@ -2,10 +2,12 @@
 Contexts
 ========
 
-Coverage.py 5.0 can record separate coverage data for different contexts during
+Coverage.py 5.0 can record separate coverage data for `different contexts`_ during
 one run of a test suite.  Pytest-cov can use this feature to record coverage
 data for each test individually, with the ``--cov-context=test`` option.
 
+.. _`different_contexts`: https://coverage.readthedocs.io/en/stable/contexts.html
+
 The context name recorded in the coverage.py database is the pytest test id,
 and the phase of execution, one of "setup", "run", or "teardown".  These two
 are separated with a pipe symbol.  You might see contexts like::
@@ -16,3 +18,12 @@ are separated with a pipe symbol.  You might see contexts like::
 
 Note that parameterized tests include the values of the parameters in the test
 id, and each set of parameter values is recorded as a separate test.
+
+To view contexts when using ``--cov-report=html``, add this to your ``.coveragerc``::
+
+    [html]
+    show_contexts = True    
+   
+The HTML report will include an annotation on each covered line, indicating the
+number of contexts that executed the line. Clicking the annotation displays a 
+list of the contexts.