Update README configuration filepath

[spokerman12]

Hello!

On the README, it says:

by putting this settings in lms/env/private.py

Seeing that settings are now in different files, could we update it to something like:

by putting this settings in /edx/etc/lms.yml (or /edx-platform/lms/envs/common.py?)

(This was tested and it works as expected on a devstack)

I'd love to make the PR but maybe the change should include both paths? I prefer asking firsthand.

Anyways, thank you for your work!!