DOC: Advise users to delete the PR template in message.

[jhlegarreta]

Improve the PR template message to highlight the fact that it is a set of
guidelines that should be removed for the final PR message.

[jhlegarreta]

Folks, when the transition to GitHub was triggered many of the commit message contained only the template. I guess they were somehow automatically created.

Although the Insights, sec. Community of GitHub does encourage to have a PR template and I added it myself to ITK, I am not satisfied with the result: I believe it adds another action to the PR if we want to keep the message clean.

Until GitHub does a better job displaying the message outside of the commit message, this PR just adds a notice to users to remember to delete the template text.

Suggestions or modifications are welcome.

[thewtex]

Thanks for your attention to this @jhlegarreta .

Another thought: maybe we want to put the file in .github/PULL_REQUEST_TEMPLATE.md as described here:

https://blog.github.com/2016-02-17-issue-and-pull-request-templates/

?

[jhlegarreta]

Will move them in a separate topic. Thanks @thewtex.

[jhlegarreta]

The mentioned files are moved in #161.

[thewtex]

Thanks, @jhlegarreta !

The content-check is requesting a newline at the end of the file -- please go ahead with the merge after addressing this.

[jhlegarreta]

16a81bb adds the newline. Thanks for the heads-up Matt.

[jhlegarreta]

The CircleCI build error is unrelated:
itkImageRandomNonRepeatingIteratorWithIndexTest (Failed)

Iterator in priority region test failed
[17, 19, 17] is outside the region (should be in)ImageRegion (0x7ffd1b8d1100)
  Dimension: 3
  Index: [0, 0, 0]
  Size: [50, 50, 50]

Merging.