Write a packager's guide

[vytas7]

It seems that people packaging Falcon for various OSes such as Linux distributions (or just hobby projects) have very different ideas how the process should look like, although PEP 517 was supposed to simplify that.

For instance, how do I test the built binary? It seems some packagers opt to invoke pytest directly without parameters and expect it to work, disregarding our tox environments completely (see: #2080, #2006 etc).

[CaselIT]

Where should this documentation be? A section in readme or a separate file?
@kloczek do you have suggestions?

[vytas7]

Absent better ideas, maybe README could link to a section on RtD.