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
Merge pull request #104 from Gazler/documentation/port-strings

Documentation: Include notes on mapping ports
Improve error when service is not a dict

Fixes #127
Reword port warning in YAML reference, remove it from README
Reformat comments in YAML reference for readability
Merge pull request #128 from orchardup/better-error-message-when-service-is-not-dict

Improve error when service is not a dict
Merge pull request #78 from orchardup/document-docker-version

Document which versions of Docker Fig supports
Document link aliases
Fix: image-defined entrypoint not overridden by intermediate container

This was causing recreation to hang.
Merge pull request #130 from orchardup/fix-hanging-recreate

Fix hanging recreate
Ship 0.3.0
Merge pull request #131 from orchardup/ship-0.3.0

Ship 0.3.0
Remove Python 3 from Travis

Let's not kid ourselves.
Add steps for contributing to README