contribution instructions

In order to encourage more contribution, should write up more detailed suggestions for contributing to Qiwi.

Include:

• suggestions for using GitLab to create new branch and submit pull request
• instructions for setting up dev environment:
  • setting up venv and installing dependencies (including pyinstaller)
  • build instructions for executables
    • generating correct pyinstaller specs (i.e. the weird thing around needing to add "images")
    • submitting/uploading to Nextcloud
• labeling system for Issues