Add CSS source maps reference to docs. Closes #15211

[peterblazejewicz]

This rather trivial PR adds relevant information about CSS source map to "What's included" documentation section and it:

• updates distribution file list with relevant files
• adds a short reference to CSS source map purpose.

bootstrap/
├── css/
│   ├── bootstrap.css
│   ├── bootstrap.css.map
│   ├── bootstrap.min.css
│   ├── bootstrap-theme.css
│   ├── bootstrap-theme.css.map
│   └── bootstrap-theme.min.css

[...] and minified CSS and JS (bootstrap.min.*) and CSS source maps (bootstrap.*.map) for use with web browsers developer tools. 
Fonts from Glyphicons are included, as is the optional Bootstrap theme. [...]

This PR is not trying to make documentation capable of fully answering questions like this one:
http://stackoverflow.com/questions/21504611/what-are-the-map-files-used-for-in-bootstrap-3-1

[cvrebert]

Perhaps we should hyperlink the term "source maps" to https://developers.google.com/chrome-developer-tools/docs/css-preprocessors (or to similar external docs).

[peterblazejewicz]

@cvrebert can/should I update PR with info callout block?
http://jsbin.com/nekoramemu/

[cvrebert]

A callout seems like overkill, and we're trying to cut down on callouts generally.
But let's definitely hyperlink the term.

[peterblazejewicz]

PR updated, now sample shows paragraph from docs:
http://jsbin.com/tidoma/

[mdo]

Merged the changes and made a few copy edits. Thanks!