Document what to do with this security issue

[stpaultim]

Security notice: Backup and Migrate will not save backup files to the server because the destination directory is publicly accessible.

I will try to document this further. Just creating a ticket to remind me.

[stpaultim]

I added some text here:
https://github.com/backdrop-contrib/d2b_migrate/wiki/Know-Issues-with-D2B-Migrate-Module

I would appreciate thoughts on content, placement and even the intent of this documentation. Is this helpful or how can we make it more helpful?