Add example how to enable extension

examples/enable-extension/Dockerfile

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+FROM registry.redhat.io/rhel8/postgresql-12
+
+ADD . $APP_ROOT/src/

examples/enable-extension/README.md

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+Extending PostgreSQL image by enabling extension
+================================================
+
+This is an example how to use the feature of extending the image (see more at https://github.com/sclorg/postgresql-container/tree/generated/12#extending-image) to enable extension `pg_stat_statements`.
+
+To use this in a Dockerfile, run:
+
+```
+podman build . -t my_postgresql:12
+```
+
+Then, run the resulting image as usually:
+
+```
+podman run -d -e POSTGRESQL_ADMIN_PASSWORD=password my_postgresql:12
+```
+
+And see the extension is enabled:
+```
+podman exec -ti -l bash
+bash-4.4$ psql
+psql (12.1)
+Type "help" for help.
+
+postgres=# \dx
+                 List of installed extensions
+  Name   | Version |   Schema   |         Description          
+---------+---------+------------+------------------------------
+ plpgsql | 1.0     | pg_catalog | PL/pgSQL procedural language
+(1 row)
+```

examples/enable-extension/postgresql-cfg/s2i-extending.conf

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+shared_preload_libraries='pg_stat_statements'

examples/enable-extension/postgresql-init/set_passwords.sh

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+#!/bin/bash
+
+echo "CREATE EXTENSION pg_stat_statements;" | psql