Add pyapp plugin.

[Kyle Anthony Williams]

Is your feature request related to a problem? Please describe

N/A

Describe the proposed solution

Add @amrox's pyapp plugin: https://github.com/amrox/asdf-pyapp/ to the README.

Describe similar asdf features and why they are not sufficient

N/A

Describe other workarounds you've considered

N/A

[Kyle Anthony Williams]

I've done some thinking. Due to how pyapp works, I don't think it would be appropriate to add it to the plugin database. Still deserves some attention, though.