README fixes (#464)

* Fix typos and use consistent YAML data structures for env

* Add Debug flag and more structure to the README

* Clarify default value of MM_HOSTNAME

* Fix typo

* Fix typos

docker-compose.yaml

@@ -46,9 +46,9 @@ services:
 
   database:
     environment:
-      POSTGRES_DB: mailmandb
-      POSTGRES_USER: mailman
-      POSTGRES_PASSWORD: mailmanpass
+    - POSTGRES_DB=mailmandb
+    - POSTGRES_USER=mailman
+    - POSTGRES_PASSWORD=mailmanpass
     image: postgres:9.6-alpine
     volumes:
     - /opt/mailman/database:/var/lib/postgresql/data