feat(postgresql): add ansible role for postgresql deployment

roles/postgresql/README.md

@@ -3,6 +3,24 @@
 PostgreSQL is the self proclaimed "world's most advanced" open source relational
 database. This ansible role can deploy and configure postgresql.
 
+By default, the role configures the remote's effective ansible user with
+peer authentication for the (postgresql) role `postgres` on all databases (with all grants).
+
+## Required configuration
+
+Set `postgresql_superuser_password` to your superusers desired password.
+
+## Optional configuration
+
+Set `postgresql_major_version` to your desired postgresql major version,
+for supported major versions see [`defaults/main/main.yml`](defaults/main/main.yml#L6).
+
+This role can be executed multiple times with different
+`postgresql_major_version` values to provide new database versions for up-to-
+date applications and older versions for software which does not yet support
+them. Container name and host mounts encode the major version to prevent
+accidental usage of the 'wrong' `PGDATA` directory.
+
 ## Requirements
 
 - `psycopg2` (pip) package