Add install instructions for ansible-relayor

There are several ways to install external roles, ie via ansible-galaxy,
git submodules, etc. I found that adding this role to a requirements.yml
file and adding instructions to the README.md is probably the best way.

Using ansible-galaxy actually had issues because the ansible-relayor git
repository has version tags that use mixed styles, like v0.3.3 and 0.1.0
without a v.

README.md

@@ -19,6 +19,10 @@ Once you've satisfied the the above assumptions, you can execute:
 
     $ ansible-playbook web.yml
 
+If setting up Tor relays, you need to install the [ansible-relayor](https://github.com/nusenu/ansible-relayor) role:
+
+    $ ansible-galaxy install -r requirements.yml
+
 ## Todo
 
 - Update packages for Ubuntu 18.04 (mariadb, nginx, tarsnap currently using packages for 17.10 artful)