- Correct the path to the compiled Next binary
- Code to reveal which pages are avaliable within an example
- Preferred way of testing an external application with a locally compiled Next
- Add markdown code annotations

As per https://twitter.com/pierrevanmart/status/1109030783405248513

The current contributing.md instructions are outdated since the project uses yarn.

The problem:
Without installing yarn we have the following error/output:

`lerna ERR! yarn run prepublish exited ENOENT in 'next-server'`

Adding new instructions on how to install and run the project.

- Implements Lerna
- Moves all source code into `packages/next`
- Keeps integration tests in the root directory

* Add contributing.md

* Only reload example app on server changes