Add support for Synapse Simple Antispam

Fixes #255 (Github Issue).

docs/configuring-playbook-synapse-simple-antispam.md

@@ -0,0 +1,16 @@
+# Setting up Synapse Simple Antispam (optional, advanced)
+
+The playbook can install and configure [synapse-simple-antispam](https://github.com/t2bot/synapse-simple-antispam) for you.
+
+See that project's documentation to learn what it does and why it might be useful to you.
+In short, it lets you fight invite-spam by automatically blocking invitiations from a list of servers specified by you (blacklisting).
+
+If you decide that you'd like to let this playbook install it for you, you need some configuration like this:
+
+```yaml
+matrix_synapse_ext_spam_checker_synapse_simple_antispam_enabled: true
+
+matrix_synapse_ext_spam_checker_synapse_simple_antispam_config_blocked_homeservers:
+- example.com
+- another.com
+```

docs/configuring-playbook.md

@@ -72,6 +72,8 @@ When you're done with all the configuration you'd like to do, continue with [Ins
 
 - [Setting up the LDAP password provider module](configuring-playbook-ldap-auth.md) (optional, advanced)
 
+- [Setting up Synapse Simple Antispam](configuring-playbook-synapse-simple-antispam.md) (optional, advanced)
+
 - [Setting up Matrix Corporal](configuring-playbook-matrix-corporal.md) (optional, advanced)