chore: add GitHub templates

[AngryMaciek]

There are several GitHub-specific text files which could help others interact with this repository: templates for issues and pull requests. It would be nice to add them here as well.

Take a look at how I set them up in one of my repos we develop in our group: bindz.
You need to add a top-level file: PULL_REQUEST_TEMPLATE.md as well as two files under .github/ISSUE_TEMPLATE. You may copy and use the same text as I do or adjust the contents to whatever you like.