Add a GitHub PR template

[hristog]

This PR proposes the introduction of a PR template, following the same structure as those in dask and distributed.

[hristog]

Hmm the "CI / docs (pull_request)" failure is new:

Updating 'dask-ml-docs' env from conda env update...
  /usr/share/miniconda3/condabin/conda env update --name dask-ml-docs --file /tmp/environment-patched.yml
  Collecting package metadata (repodata.json): ...working... done
  Solving environment: ...working... failed
  Warning: 
  ResolvePackageNotFound: 
    - python[version='3.6.6.*,3.7.*']

  ResolvePackageNotFound: 
    - python[version='3.6.6.*,3.7.*']

It doesn't seem to me that it's related to the most recent commit pushed to this PR.

[hristog]

I can see the same failure in https://github.com/dask/dask-ml/pull/813.

@jrbourbeau, could this by any chance be related to https://github.com/dask/distributed/pull/4647#issuecomment-809614689?

[jrbourbeau]

Ah, thanks for pointing the documentation build errors out -- should be fixed by https://github.com/dask/dask-ml/pull/815