add "What's Bootlint?" section to README

Refs #28

Author: cvrebert

README.md

@@ -4,7 +4,15 @@
 [![Dependency Status](https://david-dm.org/cvrebert/bootlint.svg)](https://david-dm.org/cvrebert/bootlint)
 [![devDependency Status](https://david-dm.org/cvrebert/bootlint/dev-status.svg)](https://david-dm.org/cvrebert/bootlint#info=devDependencies)
 
-HTML linter for Bootstrap projects
+An HTML [linter](http://en.wikipedia.org/wiki/Lint_(software)) for [Bootstrap](http://getbootstrap.com) projects
+
+## What's Bootlint?
+Bootlint is a tool that checks for several common HTML mistakes in webpages that are using [Bootstrap](http://getbootstrap.com) in a fairly "vanilla" way. Vanilla Bootstrap's components/widgets require their parts of the DOM to conform to certain structures. Bootlint checks that instances of Bootstrap components have correctly-structured HTML. Optimal usage of Bootstrap also requires that your pages include certain `<meta>` tags, an HTML5 doctype declaration, etc.; Bootlint checks that these are present.
+
+### Caveats
+Bootlint assumes that your webpage is already valid HTML5. If you need to check HTML5 validity, we recommend tools like [`vnu.jar`](https://github.com/validator/validator.github.io), [grunt-html](https://www.npmjs.org/package/grunt-html), or [grunt-html-validation](https://www.npmjs.org/package/grunt-html-validation).
+
+Bootlint assumes that you are using Bootstrap's default class names in your webpage, as opposed to taking advantage of the "mixins" functionality of Less or Sass to map them to custom class names. If you are using mixins, Bootlint may report some false-positive warnings. However, there are some Bootlint checks that are applicable even if you are using mixins pervasively.
 
 ## Getting Started
 ### On the command line
@@ -29,11 +37,9 @@ In your webpage:
 Then check the JavaScript console for lint warning messages.
 
 ## API Documentation
-
 Bootlint is a CommonJS module.
 
 ### Browser
-
 Bootlint exports a `bootlint` property on the global `window` object.
 In a browser environment, the following public APIs are available: