Add reason-mode link for emacs, and fix anchor links

[acchou]

Issue #22

Add a link to reason-mode for emacs and document how to run emacs within the environment of a ReasonProject. Also document how to invoke emacs appropriately on MacOS (this tripped me up personally -- it's pretty uncommon for users to be using the builtin MacOS emacs, which is unfortunately what they will get with a naive command line).

Also fix anchor links, which weren't working on github, and a header or two.

[acchou]

I just noticed that Issue #20 also addresses the broken links. This pull request is a superset of that one.

[jordwalke]

There's an index.html page too, that is just a manual copy of the markdown IIRC. Did you see that? (Be careful not to trim trailing whitespace - some of them are there intentionally to get the html page to render differently).

[acchou]

Ok, I've copied the updated contents from the readme to index.html.