Add deploy steps to contribution guide

[jwaa]

Is your feature request related to a problem? Please describe.
It is quite cumbersome to deploy a new MATRX version. It would be nice if everything you need would be in one spot.

Describe the solution you would like
An additional section in the contribution guide that describes step by step how to deploy a new MATRX version:

1. Merge with master
2. Upload it to test pypi
3. Test it (e.g. vis_test, bw4t etc)
4. If everything works, tag the version
5. Create release notes
6. Upload to pypi

Describe alternatives you have considered
Instead of doing it manually we can use CI, but no idea how that works.

Additional context
N/A

[thaije]

In addition, adding the actual git commands would be nice for step 4:
git pull (latest master branch you pushed)
git tag v_YOUR_VERSION -a
git push --tags

How to install from test server:
See version on test pypi: https://test.pypi.org/project/matrx/1.1.0a1/

[jwaa]

Done!