Documentation: Include notes on mapping ports

When mapping ports as strings there is an issue with the way YAML parses
numbers in the format "xx:yy" where yy is less than 60 - this issue is
now included in the documentation.

This fixes #103

docs/wordpress.md

@@ -26,7 +26,7 @@ web:
   build: .
   command: php -S 0.0.0.0:8000 -t /code
   ports:
-    - 8000:8000
+    - "8000:8000"
   links:
     - db
   volumes:
@@ -34,7 +34,7 @@ web:
 db:
   image: orchardup/mysql
   ports:
-    - 3306:3306
+    - "3306:3306"
   environment:
     MYSQL_DATABASE: wordpress
 ```