docs: Add to Docker setup (apache#1204)

- add command to enter psql interface from command line.

(cherry picked from commit 25660e5)

Author: markgomer

README.md

@@ -196,6 +196,12 @@ docker run \
     apache/age
 ```
 
+<h5> Enter PostgreSQL's psql: </h5>
+
+```bash
+docker exec -it age psql -d postgresDB -U postgresUser
+```
+
 <h2><img height="20" src="/img/contents.svg">&nbsp;&nbsp;Post Installation</h2>
 
 For every connection of AGE you start, you will need to load the AGE extension.