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

README.md

@@ -154,7 +154,8 @@ the host running the containers and are imported at runtime in the containers.
   is imported by the [existing configuration][2]
   provided by the mailman-web container. **This file is referred to as 
   `settings.py` in most of the Postorius and Django documentation.** To change
-  or override any settings in Django/Postorius, you need to create/edit this file. 
+  or override any settings in Django/Postorius, you need to create/edit this file.
+  A useful configuration for troubleshooting is `DEBUG = True`.
 
 [2]: https://github.com/maxking/docker-mailman/blob/master/web/mailman-web/settings.py
 
@@ -190,7 +191,7 @@ environment variables mentioned above (`MAILMAN_ADMIN_USER` &
 password, plese follow the "Forgot Password" link on the "Sign In" page.
 
 To configure the mailman-web container to send emails, add this to your
-`settings_local.py`.:
+`settings_local.py`:
 
 ```
 EMAIL_BACKEND = 'django.core.mail.backends.smtp.EmailBackend'
@@ -333,11 +334,13 @@ The provided docker containers do not have an MTA in-built. You can either run
 your own MTA inside a container and have them relay emails to the mailman-core
 container or just install an MTA on the host and have them relay emails.
 
+### Exim4
+
 To use [Exim4][8], it should be setup to relay emails from mailman-core and
 mailman-web. The mailman specific configuration is provided in the
 repository at `core/assets/exim`. There are three files
 
-- [25_mm_macros](core/assets/exim/25_mm3_macros) to be placed at
+- [25_mm3_macros](core/assets/exim/25_mm3_macros) to be placed at
   `/etc/exim4/conf.d/main/25_mm3_macros` in a typical Debian install of
   exim4. Please change MY_DOMAIN_NAME to the domain name that will be used to
   serve mailman. Multi-domain setups will be added later.
@@ -364,6 +367,8 @@ smtp_port: $SMTP_PORT
 configuration: python:mailman.config.exim4
 ```
 
+### Postfix
+
 To use [Postfix][12], edit the `main.cf` configuration file, which is typically
 at `/etc/postfix/main.cf` on Debian-based operating systems.  Add
 mailman-core and mailman-web to `mynetworks` so it will relay emails from