Add a document describing how p4a interacts with pip & python packages

[ghost]

This pull request adds an internal documentation (not build as part of user docs, I just put it in there so we can find it again - it's mainly meant to just sit around in a place where we can find it) describing how p4a interacts with pip & python packages. This is mainly meant for pip developers and others outside of our team to be able to get an idea what we're doing, although of course it could also be interesting to newcomers to our own team!

Please have fun reading and tell me if anything doesn't make sense!

[inclement]

Looks quite complete to me, nice.

Although I would add full stops in a lot of places :<

[inclement]

Oh, and as for where this goes, I don't think it would be inappropriate to put it in the documentation. It could be e.g. a subpage of "Development and Contributing". I don't think that would cause issues, and it would be nice to have it be discoverable and easily referenceable in the compiled documentation.

[ghost]

@inclement good point! I added it into the "Development and Contributing" section now. will also make it easier to link once it's out on read the docs

[AndreMiras]

This documentation is a good addition, thanks.
I only saw a minor typo and also the missing full stops also bugged me 😄
In general what p4a is doing with the recipes reminds me what Gentoo (source distribution) is doing with Ebuilds. Ebuild basically has the same features, dependencies management, patching, build tools...

[AndreMiras]

LGTM