Document HMBKP_PATH in FAQ section of README

Hi, 

This documents the one feature that I briefly thought was missing when I first installed the plugin. When I looked into it to add it in, it turns out that it's already possible. So - this PR documents the usage of HMBKP_PATH in the readme. 

Not sure if you auto-generate the readme.txt from the .md or vice versa, so changed both.