Merge pull request #220 from chainer/muupan-patch-1

Update the contribution guide to use pytest

Author: muupan

CONTRIBUTING.md

@@ -11,7 +11,7 @@ If you could kindly send a PR to ChainerRL, please make sure all the tests succe
 
 ## Testing
 
-To test chainerrl modules, install `nose` and run `nosetests`. Pass `-a '!gpu'` to skip tests that require gpu.
+To test chainerrl modules, install and run `pytest`. Pass `-m "not gpu"` to skip tests that require gpu.
 
 To test examples, run `test_examples.sh [gpu device id]`. `-1` would run examples with only cpu.