Add a custom resource to set up PostgreSQL 12

Supports both primary and replica. The access rules and firewall have to
be set up outside of the custom resource, so they are part of the
recipes instead

Refs #160

site-cookbooks/kosmos-postgresql/README.md

@@ -1,4 +1,41 @@
 # kosmos-postgresql
 
-TODO: Enter the cookbook description here.
+## Custom resources
 
+### `postgresql_custom_server`
+
+Usage:
+
+When the `tls` attribute is set to true, a TLS certificate for the FQDN
+(`node['fqdn']`, for example `andromeda.kosmos.org`) is generated using Let's
+Encrypt and copied to the PostgreSQL data directory and added to the
+`postgresql.conf` file
+
+#### On the primary:
+
+```ruby
+postgresql_custom_server "12" do
+  role "primary"
+  tls true
+end
+```
+
+#### On a replica:
+
+```ruby
+postgresql_custom_server "12" do
+  role "primary"
+  tls true
+end
+```
+
+After the initial Chef run on the replica, run Chef on the primary to add the
+firewall rules and PostgreSQL access rules, then run Chef again on the replica
+to set up replication.
+
+#### Caveat
+
+[`firewall_rules`](https://github.com/chef-cookbooks/firewall/issues/134) and
+[`postgresql_access`](https://github.com/sous-chefs/postgresql/issues/648) need
+to be declared in recipes, not resources because of the way custom resources
+work currently in Chef