Suggestion: explain Pages a tiny bit more in readme intro

[brittag]

Currently the intro says "It works very similarly to GitHub pages" - it could be helpful to also explain how it's different from GitHub Pages. :) For readers who are potentially interested in reusing Pages within their own teams/orgs/agencies, it could be helpful to explain the reasons why (or why not) to choose 18F Pages vs GitHub Pages. (This might be as simple as linking to this blog post that explains that GitHub Pages doesn't support things we need for .gov sites, if those issues are still relevant.)

[mbland]

Just assigned myself, @brittag. Expect PRs soon. :smiley: