Discuss how hhi files are supported with respect to the typechecker

Although `hh_client` can be used directly on the command line, having the immediate feedback directly in an editor can be an amazing productivity boost. If your preferred editor or IDE is unsupported, contributions of additional plugins are very welcome; the existing plugins are decent examples of how to script the `hh_client` interface.

## Nuclide

The [Nuclide editor](http://nuclide.io/) has first-class support for Hack. It not only provides inline typechecking, but it also supports autocomplete, jump-to-definition and project search.

## Vim

 Vim users can find a package in [Github](https://github.com/hhvm/vim-hack) with installation instructions in the [README](https://github.com/hhvm/vim-hack/blob/master/README.md) contained therein.

## Emacs

Emacs users will find a plugin inside `/usr/share/hhvm/hack/emacs`, when installing Hack from a supported HHVM package.

Add documentation for editor support
Add documentation for editor support
