Adding README.rst entries for how to use buildozer with python3

[andyDoucette]

Thanks to rafalo13 and inclement for suggestions on how to get this to work. I'm just documenting it for others to use.

[ghost]

One thing I don't like is the information about p4a.local_recipes. Bugs should be resolved in code, not in the documentation. You should not add informations about how to fix bug which will be resolved in the next few days. Also, p4a.local_recipes error is not related to the python3 support in the toolchain. The rest is good 👍

[andyDoucette]

If bug fixes in code are an option, that's even better! :) I invite you to edit my pull request as desired and take what you like from it. Sounds like you'll just be removing a couple lines and delaying the update until after the bug is fixed? Sounds good to me.

[Albert-Gao]

Thanks for this, I currently use the official approach which use the hostpython2 to package, my code is written in python3, will it be any problem?

[tito]

Thanks you