Create PR & Issue Templates, move CONTRIBUTING.md

[jpommerening]

GitHub just announced a new feature enabling repos to have templates for pull requests and issues:
https://github.com/blog/2111-issue-and-pull-request-templates

Basically, all we need to do is create the ISSUE_TEMPLATE.md and PULL_REQUEST_TEMPLATE.md files and commit them, similar to the CONTRIBUTING.md. Additionally, we may move these files into a .github sub directory, but it looks like this is not mandatory.

@LaxarJS/committers, @mkemmann: Let's decide on what info we need from issue and pull request authors. We already listed the requirements in the CONTRIBUTING.md so this part is mostly bikeshedding on the format. Also, files in the repo root keep adding up, should we use the subdirectory?

[jpommerening]

My 2 ¢: All I want for christm—, err, PRs is a checkbox that says:

• I have read and signed the CLA.

Because checkboxes are fun 🎈

[alex3683]

💌 .github subdirectory