help contributors be more productive

certainly saved me time after I found this in hack/make.sh :)

Docker-DCO-1.1-Signed-off-by: Tim Ruffles <oi@truffles.me.uk> (github: github_handle)

docs/sources/contributing/devenvironment.rst

@@ -122,7 +122,10 @@ If the test are successful then the tail of the output should look something lik
 	PASS
 	ok  	github.com/dotcloud/docker/utils	0.017s
 
+If $TESTFLAGS is set in the environment, it is passed as extra arguments to 'go test'.
+You can use this to select certain tests to run, eg.
 
+    TESTFLAGS='-run ^TestBuild$' make test
 
 
 Step 6: Use Docker