Added info about check-manifest

CONTRIBUTING.rst

@@ -134,6 +134,16 @@ Testing the source code vs. the package
 
 When running `pytest`, you are testing the source code in the current `venv`. When running `tox`, you are testing the package generated by `python setup.py sdist`. It's important to run `tox` as it runs test against the package other users will install. `tox` can fail even if `pytest` succeeds because of an incorrect `MANIFEST.in` file or missing dependencies within the `setup.py` `install_requires` argument.
 
+Checking MANIFEST.in
+--------------------
+
+It's easy to add files to version control but forget to include in the MANIFEST.in file. After committing changes, run the following to see if any files are missing:
+
+    $ check-manifest
+
+
+
+
 Deploying
 ---------