Create PLEP for defining ADDON namespace packages

[rocco8773]

Create a PLEP that defines the scope of packages for the plasmapy.addons namespace package.

Things that could be needed...

• Define the motivation namespace packages
• Define the scope/decision criteria for creating a distributed package that creates a PlasmaPy namespace package
• Define the structure of a distribution package that would add a sub-package/module to the plasmapy.addons namespace package.
• PlasmaPy's website should broadcast "Registered/Affiliated/Addon" packages
  • "Registered/Affiliated" packages would not be namespace packages but other packages that work nicely (? what's nicely) with PlasmaPy
  • In order to be an authorized/broadcasted "addon" package, a PlasmaPy Committee member must be a maintainer of the package.
  • An "addon" package does not need to be hosted on the PlasmaPy GitHub repository
  • User can still create their own "addon" packages, but they will not be advertised by PlasmaPy unless a Committee member is a maintainer.