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/index.md

@@ -21,7 +21,7 @@ web:
   links:
    - db
   ports:
-   - 8000:8000
+   - "8000:8000"
 db:
   image: orchardup/postgresql
 ```
@@ -107,7 +107,7 @@ We then define a set of services using `fig.yml`:
       build: .
       command: python app.py
       ports:
-       - 5000:5000
+       - "5000:5000"
       volumes:
        - .:/code
       links: