Update sass.md (#710)

* Update sass.md

Make `Mixin Properties` a subpart of `Mixins`, so that they are displayed next to each other, since they are a logical unit.
Add section about default values and default variables for mixins.

* Update sass.md according to comments in #710

sass.md

@@ -66,7 +66,7 @@ h1 {
 }
 ```
 
-### Mixin properties
+#### with parameters
 
 ```scss
 @mixin font-size($n) {
@@ -80,6 +80,39 @@ body {
 }
 ```
 
+#### with default values
+
+```scss
+@mixin pad($n: 10px) {
+  padding: $n;
+}
+```
+
+```scss
+body {
+  @include pad(15px);
+}
+```
+
+#### with a default variable
+
+```scss
+// Set a default value
+$default-padding: 10px;
+```
+
+```scss
+@mixin pad($n: $default-padding) {
+  padding: $n;
+}
+```
+
+```scss
+body {
+  @include pad(15px);
+}
+```
+
 ### Extend
 
 ```scss