Merge pull request #15306 from vsn4ik/whats_included_readme

Update source maps info in README.md.

README.md

@@ -39,8 +39,10 @@ Within the download you'll find the following directories and files, logically g
 bootstrap/
 ├── css/
 │   ├── bootstrap.css
+│   ├── bootstrap.css.map
 │   ├── bootstrap.min.css
 │   ├── bootstrap-theme.css
+│   ├── bootstrap-theme.css.map
 │   └── bootstrap-theme.min.css
 ├── js/
 │   ├── bootstrap.js
@@ -52,7 +54,7 @@ bootstrap/
     └── glyphicons-halflings-regular.woff
 ```
 
-We provide compiled CSS and JS (`bootstrap.*`), as well as compiled and minified CSS and JS (`bootstrap.min.*`). Fonts from Glyphicons are included, as is the optional Bootstrap theme.
+We provide compiled CSS and JS (`bootstrap.*`), as well as compiled and minified CSS and JS (`bootstrap.min.*`). CSS [source maps](https://developers.google.com/chrome-developer-tools/docs/css-preprocessors) (`bootstrap.*.map`) are available for use with certain browsers' developer tools. Fonts from Glyphicons are included, as is the optional Bootstrap theme.