Add default PR description

[AbdealiLoKo]

I was submitting a patch at scikit-image and I was really impressed with their default PR message:

## Description
[Tell us about your new feature, improvement, or fix! If relevant, please supplement the PR with images.]


## Checklist
[It's fine to submit PRs which are a work in progress! But before they are merged, all PRs should provide:]
- [ ] Clean style in [the spirit of PEP8](https://www.python.org/dev/peps/pep-0008/)
- [ ] [Docstrings for all functions](https://github.com/numpy/numpy/blob/master/doc/example.py)
- [ ] Gallery example in `./doc/examples` (new features only)
- [ ] Unit tests

[For detailed information on these and other aspects see [scikit-image contribution guidelines](http://scikit-image.org/docs/dev/contribute.html)]


## References
[If this is a bug-fix or enhancement, it closes issue # ]
[If this is a new feature, it implements the following paper: ]

The checklist is really nice and serves as a better reminder of "things to do" as compared to writing it in a documentation.

[gitmate-bot]

Thanks for reporting this issue!

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CC @sils1297, @AbdealiJK

[AbdealiLoKo]

You can find their other templates and this one at https://github.com/scikit-image/scikit-image/tree/master/.github

[adtac]

We could also add something about doctests. Also modify the first check point to something like "Run coala on the project before submitting" as that should pretty much cover any code standards issue. (We could also convince scikit-image devs to use coala.)

[Adrianzatreanu]

This issue was moved to coala/documentation#31