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/rails.md

@@ -30,14 +30,14 @@ Finally, `fig.yml` is where the magic happens. It describes what services our ap
     db:
       image: orchardup/postgresql
       ports:
-        - 5432
+        - "5432"
     web:
       build: .
       command: bundle exec rackup -p 3000
       volumes:
         - .:/myapp
       ports:
-        - 3000:3000
+        - "3000:3000"
       links:
         - db