Document POSTGRES_INITDB_ARGS env var

This documents the POSTGRES_INITDB_ARGS environment variable added in #132.

postgres/content.md

@@ -53,6 +53,10 @@ This optional environment variable can be used to define another location - like
 
 This optional environment variable can be used to define a different name for the default database that is created when the image is first started. If it is not specified, then the value of `POSTGRES_USER` will be used.
 
+### `POSTGRES_INITDB_ARGS`
+
+This optional environment variable can be used to send arguments to `postgres initdb`. The value is a space separated string of arguments as `postgres initdb` would expect them. This is useful for adding functionality like data page checksums: `-e POSTGRES_INITDB_ARGS="--data-checksums"`.
+
 # How to extend this image
 
 If you would like to do additional initialization in an image derived from this one, add one or more `*.sql` or `*.sh` scripts under `/docker-entrypoint-initdb.d` (creating the directory if necessary). After the entrypoint calls `initdb` to create the default `postgres` user and database, it will run any `*.sql` files and source any `*.sh` scripts found in that directory to do further initialization before starting the service.