Add clarification and example for matrix strategy

[rytswd]

Fixes #27

• A simple example job has been added (which will be run every time a commit is pushed to master)
• A quick documentation has been added for matrix use case

Side Note After writing the doc above, I realise how "ignore" setup would probably need to support a bit more sophisticated input such as regex - otherwise it would be really difficult to use when you want to test an extra matrix strategy and do not care errors from those. I will leave that as a separate ticket.