add best practices about packaging

[jaimergp]

Description

Type of change

• Bug-fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

References

Closes napari/packaging#3

How has this been tested?

• example: the test suite for my feature covers cases x, y, and z
• example: all tests pass with my change

Final checklist:

• My PR is the minimum possible work for the desired functionality
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• If I included new strings, I have used trans. to make them localizable.
  For more information see our translations guide.

[psobolewskiPhD]

It's all clear to me, but maybe one other person could take a look?
Maybe @goanpeca or @DragaDoncila ?