GitHub PR Template

[Trevor Grant]

Looking for a Lazy Consensus to take information from

https://mahout.apache.org/developers/patch-check-list.html

and create a pull request template as described in
https://help.github.com/articles/creating-a-pull-request-template-for-your-repository/

What will happen then is when you open a PR on github, the message will
prefill to something like:
--------------
**Purpose of PR:**

Please mark each with an "x"
- [ ] A JIRA ticket exists (if not, please create this first)
- [ ] Title of PR is "MAHOUT-XXXX Breif Description of Changes" where XXXX
is the JIRA number.
- [ ] Created unit tests where appropriate
- [ ] Added licenses correct on newly added files
- [ ] Assigned JIRA to self
- [ ] Added documentation in scala docs/java docs, (and website once that
is merged to dev)
- [ ] Successfully built and ran all unit tests, verified that all tests
pass locally.
- [ ] Updated the CHANGELOG file. Give proper credit to the authors.

If all of these things aren't complete, but you still feel it is
appropriate to open a PR, please add [WIP] after MAHOUT-XXXX before the
descriptions- e.g. "MAHOUT-XXXX [WIP] Description of Change"
--------------


FYI, in markdown - [ ] is an empty check box, - [x] is a checked-checkbox.
The idea is the person creating the PR will mark all of those check boxes.
This is just a friendly reminder to the user to make sure to have done
all of these things, and will help with reviews.