Add `issue_template.md` and `pull_request_template.md`

This appears to be a new feature of GitHub.

See these links for more details:
- https://github.com/dear-github/dear-github/issues/125
- https://github.com/owncloud/core/issues/new
- https://github.com/blog/2111-issue-and-pull-request-templates

[ci skip]

.github/issue_template.md

+### Steps to reproduce
+
+(Guidelines for creating a bug report are [available
+here](http://guides.rubyonrails.org/contributing_to_ruby_on_rails.html#creating-a-bug-report)
+
+### Expected behavior
+Tell us what should happen
+
+### Actual behavior
+Tell us what happens instead
+
+### System configuration
+**Rails version**:
+
+**Ruby version**:

.github/pull_request_template.md

+### Summary
+
+Provide a general description of the code changes in your pull
+request... were there any bugs you had fixed? If so, mention them. If
+these bugs have open GitHub issues, be sure to tag them here as well,
+to keep the conversation linked together.
+
+### Other Information
+
+If there's anything else that's important and relevant to your pull
+request, mention that information here. This could include
+benchmarks, or other information.
+
+Finally, if your pull request affects documentation or any non-code
+changes, guidelines for those changes are [available
+here](http://guides.rubyonrails.org/contribution_to_ruby_on_rails.html#contributing-to-the-rails-documentation)
+
+Thanks for contributing to Rails!