[Sass] Convert the beginning of the REFERENCE to refer to SCSS as the…

… primary syntax.

doc-src/SASS_REFERENCE.md

@@ -3,20 +3,56 @@
 * Table of contents
 {:toc}
 
-Sass is a meta-language on top of CSS
-that's used to describe the style of a document
-cleanly and structurally,
-with more power than flat CSS allows.
-Sass both provides a simpler, more elegant syntax for CSS
-and implements various features that are useful
-for creating manageable stylesheets.
+Sass is an extension of CSS
+that adds power and elegance to the basic language.
+It allows you to use [variables](#variables_), [nested rules](#nested_rules),
+[mixins](#mixins), [inline imports](#import), and more,
+all with a fully CSS-compatible syntax.
+Sass helps keep large stylesheets well-organized,
+and get small stylesheets up and running quickly,
+particularly with the help of
+[the Compass style library](http://compass-style.org).
 
 ## Features
 
-* Whitespace active
-* Well-formatted output
-* Elegant input
-* Feature-rich
+* Fully CSS3-compatible
+* Language extensions such as variables, nesting, and mixins
+* Many {Sass::Script::Functions useful functions} for manipulating colors and other values
+* Advanced features like [control directives](#control_directives) for libraries
+* Well-formatted, customizable output
+* [Firebug integration](https://addons.mozilla.org/en-US/firefox/addon/103988)
+
+## Syntax
+
+There are two syntaxes available for Sass.
+The first, known as SCSS (Sassy CSS) and used throughout this reference,
+is an extension of the syntax of CSS3.
+This means that every valid CSS3 stylesheet
+is a valid SCSS file with the same meaning.
+In addition, SCSS understands most CSS hacks
+and vendor-specific syntax, such as [IE's old `filter` syntax](http://msdn.microsoft.com/en-us/library/ms533754%28VS.85%29.aspx).
+This syntax is enhanced with the Sass features described below.
+Files using this syntax have the `.scss` extension.
+
+The second and older syntax, known as the indented syntax (or sometimes just "Sass"),
+provides a more concise way of writing CSS.
+It uses indentation rather than brackets to indicate nesting of selectors,
+and newlines rather than semicolons to separate properties.
+Some people find this to be easier to read and quicker to write than SCSS.
+The indented syntax has all the same features,
+although some of them have slightly different syntax;
+this is described in {file:INDENTED_SYNTAX.md the indented syntax reference}.
+Files using this syntax have the `.sass` extension.
+
+Either syntax can [import](#import) files written in the other.
+Files can be automatically converted from one syntax to the other
+using the `sass-convert` command line tool:
+
+    # Convert Sass to SCSS
+    $ sass-convert style.sass style.scss
+
+    # Convert SCSS to Sass
+    $ sass-convert style.scss style.sass
 
 ## Using Sass
 
@@ -561,7 +597,7 @@ and the result printed out for you:
     >> #777 + #888
     white
 
-### Variables: `$`
+### Variables: `$` {#variables_}
 
 The most straightforward way to use SassScript
 is to use variables.