Clarify which tests to run in the Makefile

If you put your Virtualenv in the same folder as Requests (as I
usually do), py.test will pick up any test_* files inside the
virtualenv and attempt to run them too, leading to the following error:
https://gist.github.com/kevinburke/1aa2b07e01de3a7daa15

Instead specify the test file we would like to run in the Makefile.

Makefile

@@ -4,7 +4,9 @@ init:
 	pip install -r requirements.txt
 
 test:
-	py.test
+	# This runs all of the tests. To run an individual test, run py.test with
+	# the -k flag, like "py.test -k "
+	py.test test_requests.py
 
 coverage:
 	py.test --verbose --cov-report term --cov=requests test_requests.py