Add .htaccess file and/or instructions on how to disable web access

[emrikol]

The cache files should not be accessible at all via the web server. Adding a generic .htaccess file and/or instructions (nginx perhaps) to the README file will help users secure the cache files.

(Props to WordPress.org user @bugnumber9)

[szepeviktor]

# Apache < 2.3
<IfModule !mod_authz_core.c>
    Order Allow,Deny
    Deny from all
    Satisfy All
</IfModule>
# Apache ≥ 2.3
<IfModule mod_authz_core.c>
    Require all denied
</IfModule>