Add express-gateway docs

[XVincentX]

This PR will add the required documentation to make Express Gateway land in the official program images.

[XVincentX]

@yosifkit @tianon

Is there any way I can speed up this and make it happen?

[tianon]

A few minor comments -- also, you'll want to check Travis for markdownfmt fixes (which can be applied automatically via ./markdownfmt.sh -w express-gateway/content.md from a checkout of this repo, if you wanted to take the easy way out, but sometimes it does interesting things so it needs to be reviewed afterwards to make sure it's still what you intended). 👍

[XVincentX]

@tianon I addressed all the comments — however the markdown format checking is failing but to be honest I do not really understand its reason. Can you give me a hint?

[yosifkit]

Looks like a couple things:

• list items need to be formatted with tabs

  -	item

• italics needs to be star a * rather than underscore _ (bold should be two star)
• lines next to each other need to either have a empty line between them to be considered different paragraphs, or be one line to be a single paragraph
  • i.e., paragraphs need to be a single line and have a single empty line between paragraphs

[XVincentX]

Oh I understood only now that the error message was instead a diff that was trying to help me!

I should have fixed all the issues. @yosifkit @tianon

[XVincentX]

This is so great! Thanks a lot!