doc: add info about unique job names (#734)

Author: jesterhazy

CONTRIBUTING.md

@@ -65,6 +65,10 @@ Some of the prefixes allow abbreviation -- `break`, `feat`, `depr`, and `doc` ar
 
 For the rest of the message, use imperative style and keep things concise but informative. See [How to Write a Git Commit Message](https://chris.beams.io/posts/git-commit/) for guidance.
 
+### Integration tests
+
+Our CI system runs integration tests (the ones in the `tests/integ` directory) in parallel. If you are writing or modifying a test that creates a SageMaker job (training, tuner, or transform) or an endpoint, it's important to assign a concurrency-friendly `job_name` (or `endpoint_name`), or your tests may fail randomly due to name collisions. We have a helper method `sagemaker.utils.unique_name_from_base(base, max_length)` that makes test-friendly names. You can find examples of how to use it [here](https://github.com/aws/sagemaker-python-sdk/blob/3816a5658d3737c9767e01bc8d37fc3ed5551593/tests/integ/test_tfs.py#L37) and 
+[here](https://github.com/aws/sagemaker-python-sdk/blob/3816a5658d3737c9767e01bc8d37fc3ed5551593/tests/integ/test_tuner.py#L616), or by searching for "unique\_name\_from\_base" in our test code.
 
 ## Finding contributions to work on
 Looking at the existing issues is a great way to find something to contribute on. As our projects, by default, use the default GitHub issue labels ((enhancement/bug/duplicate/help wanted/invalid/question/wontfix), looking at any ['help wanted'](https://github.com/aws/sagemaker-python-sdk/labels/help%20wanted) issues is a great place to start.