chore: GitHub templates, Code of Conduct, Contributing instructions

[AngryMaciek]

There are several text files usually included in every high-quality GH repo:

• github templates for issues and pull requests
• instructions for conttributors
• a code of conduct for the project

It would be nice to add them here as well. All of them can be found, for example, at this repo: https://github.com/zavolanlab/bindz