doc: added information on how to run the linter.

Added clarification about the linter execution.

PR-URL: nodejs#7534
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

CONTRIBUTING.md

@@ -150,6 +150,9 @@ $ ./configure && make -j8 test
 Make sure the linter is happy and that all tests pass. Please, do not submit
 patches that fail either check.
 
+Running `make test` will run the linter as well unless one or more tests fail.
+If you want to run the linter without running tests, use `make lint`.
+
 If you are updating tests and just want to run a single test to check it, you
 can use this syntax to run it exactly as the test harness would: