Test against multiple MongoDB versions in Travis (#1074)

CONTRIBUTING.rst

@@ -29,19 +29,20 @@ Style Guide
 -----------
 
 MongoEngine aims to follow `PEP8 <http://www.python.org/dev/peps/pep-0008/>`_
-including 4 space indents. When possible we try to stick to 79 character line limits.
-However, screens got bigger and an ORM has a strong focus on readability and
-if it can help, we accept 119 as maximum line length, in a similar way as
-`django does <https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/coding-style/#python-style>`_
+including 4 space indents. When possible we try to stick to 79 character line
+limits. However, screens got bigger and an ORM has a strong focus on
+readability and if it can help, we accept 119 as maximum line length, in a
+similar way as `django does
+<https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/coding-style/#python-style>`_
 
 Testing
 -------
 
 All tests are run on `Travis <http://travis-ci.org/MongoEngine/mongoengine>`_
-and any pull requests are automatically tested by Travis. Any pull requests
-without tests will take longer to be integrated and might be refused.
+and any pull requests are automatically tested. Any pull requests without
+tests will take longer to be integrated and might be refused.
 
-You may also submit a simple failing test as a PullRequest if you don't know
+You may also submit a simple failing test as a pull request if you don't know
 how to fix it, it will be easier for other people to work on it and it may get
 fixed faster.
 
@@ -49,13 +50,18 @@ General Guidelines
 ------------------
 
 - Avoid backward breaking changes if at all possible.
+- If you *have* to introduce a breaking change, make it very clear in your
+  pull request's description. Also, describe how users of this package
+  should adapt to the breaking change in docs/upgrade.rst.
 - Write inline documentation for new classes and methods.
 - Write tests and make sure they pass (make sure you have a mongod
   running on the default port, then execute ``python setup.py nosetests``
   from the cmd line to run the test suite).
-- Ensure tests pass on every Python and PyMongo versions.
-  You can test on these versions locally by executing ``tox``
-- Add enhancements or problematic bug fixes to docs/changelog.rst
+- Ensure tests pass on all supported Python, PyMongo, and MongoDB versions.
+  You can test various Python and PyMongo versions locally by executing
+  ``tox``. For different MongoDB versions, you can rely on our automated
+  Travis tests.
+- Add enhancements or problematic bug fixes to docs/changelog.rst.
 - Add yourself to AUTHORS :)
 
 Documentation
@@ -69,3 +75,6 @@ just make your changes to the inline documentation of the appropriate
 branch and submit a `pull request <https://help.github.com/articles/using-pull-requests>`_.
 You might also use the github `Edit <https://github.com/blog/844-forking-with-the-edit-button>`_
 button.
+
+If you want to test your documentation changes locally, you need to install
+the ``sphinx`` package.