Document options to include the shariff backend library

There seems to be some documentation needed how the backend library can be set up, see for example here https://github.com/heiseonline/shariff-backend-php/issues/142 or #2963009: How to set "Shariff PHP Backend" ? → .

1. Make sure that your Webserver has access to a folder outside the drupal root directory
2. Clone the shariff-backend-php repository from https://github.com/heiseonline/shariff-backend-php.git
3. Make sure that your user and the user group www-data have access to the directory chown -Rh $LOGNAME:www:data; chmod -R 775 shariff-backend-php
4. Create a Subdomain (Shariff should not run inside Drupals Root Dir, because this could cause security issues
5. Create a Subdomain for Nginx or Apache that points to the directory or ask your Webhosting service
6. Go to yoursubdomain@yourdomain.tdl

If everything worked you should see null. Now you can Test and configure the backend as described here:

https://github.com/heiseonline/shariff-backend-php