Mention the two syntaxes in /guide (sass#631)

Closes sass#615

source/guide.html.haml

@@ -58,7 +58,17 @@ introduction: >
     Sass would watch all files in the `app/sass` folder for changes, and
     compile CSS to the `public/stylesheets` folder.
 
-    ---
+  .sl-c-callout.sl-c-callout--fun-fact
+    :markdown
+      ### 💡 Fun fact:
+
+      Sass has two syntaxes! The SCSS syntax (`.scss`) is used most commonly. It's
+      a superset of CSS, which means all valid CSS is also valid SCSS. The
+      indented syntax (`.sass`) is more unusual: it uses indentation rather than
+      curly braces to nest statements, and newlines instead of semicolons to
+      separate them. All our examples are available in both syntaxes.
+
+  %hr
 
 %section#topic-2
   :markdown