Create package development guide

[adelhult]

Is your feature request related to a problem? Please describe.
We should write a short intro guide about package development and remember to link it up to this as well:

Describe the solution you'd like
I think we can keep it pretty short if we don't have time. Just a README on github with some info about manifests etc. Basically just updating: https://github.com/modmark-org/modmark/tree/main/packages