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

README.md

@@ -22,11 +22,14 @@ web:
   links:
    - db
   ports:
-   - 8000:8000
+   - "8000:8000"
+   - "49100:22"
 db:
   image: orchardup/postgresql
 ```
 
+**Note** When mapping ports in the format HOST:CONTAINER you may experience erroneous results when using a container port lower than 60. This is due to YAML parsing numbers in the format "xx:yy" as sexagesimal (base 60) for this reason it is recommended to define your port mappings as strings.
+
 (No more installing Postgres on your laptop!)
 
 Then type `fig up`, and Fig will start and run your entire app: