Add GitHub Pull Request Template

[tkem]

There has been some noticeable increase in activity recently, and although this is welcome, of course, we should put up some basic guidelines for PRs to maintain certain quality standards and reduce overall maintenance.

For example:

1. Before submitting a PR, a corresponding Issue should be created and approved, except for simple typos and the like.
2. PRs must include corresponding unit tests and documentation. Coverage must be kept at 100%, and all tests must pass, naturally.
3. New features are still discouraged, as stated in the issue template, so the motivation must be presented clearly and convincingly.
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