Add Writefreely service

[ZenKyma]

Following #12, I finally managed to create a writefreely role.

Please feel free to comment and add requests!

[spantaleev]

I've improved the documentation a bit and this is ready to merge.

One of things I've done is mention that WriteFreely can make use of a MariaDB database, but is currently not configured to do so. I think it would be preferable if support for MariaDB (and automatic wiring with the MariaDB role via templates/group_vars_mash_servers) is added.

I suppose WriteFreely uses SQLite when not configured to use MariaDB.

Most other services which can use a database (mostly Postgres, but sometimes MariaDB) are developed in a way to make use of it. If you're feeling like adding MariaDB support in a future PR, that'd be great!

[ZenKyma]

Thank you @spantaleev ! I was indeed considering to add support for MariaDB sometime in the future when I have the time, but probably I will need some help in connecting all the dots with that. I'll open a draft pr as soon as I start working on it.