Document exposed middleware

[kennethormandy]

This PR was merged, I think we can pretty much copy the docs from @jvandemo’s thorough description sintaxi/harp#454

[jvandemo]

@kennethormandy — Thanks for merging! Let me know if I can help with documentation.

[kennethormandy]

If you want to open a PR for this, we’ll definitely accept it! This would be the right page: https://github.com/harp/harpjs.com/blob/master/public/docs/environment/lib.md

[jvandemo]

@kennethormandy — Ok, I will make it happen. As soon as the page is ready, I'll create a PR and reference it in this thread. Thanks!

[jvandemo]

@kennethormandy — PR has been created: #56.

Feel free to contact me if you feel additions or changes are needed.

Thanks!