lang: python: readme: document mechanism for corner-case python[3]-package.mk location

lang: python: readme: document mechanism for corner-case python[3]-package.mk location

After some thinking over this, documenting this behavior makes sense
versus adding some functionst to handle this.
There is some validity/use-cases where some users may want to reference
a python[3]-package.mk from some other location as well as have the
flexibility to change it (locally). One example can be when the local
`packages` is renamed to something else.

This does not fall on the responsibility of the Python maintainers, but
it can be documented.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>