minScrollbarLength setting

I couldn't figure out the rebasing stuff so I just deleted my fork and
redid the work.  I believe I followed the contributing guidlines
correctly as well.

README.md

@@ -62,7 +62,10 @@ Default: 10
 If this option is true, when the scroll reach the end of the side, mousewheel event will be propagated to parent element.  
 Default: false
 
-
+### minScrollbarLength
+When set to an integer value, the thumb part of the scrollbar will not shrink below that number of pixels
+Default: null
+ 
 How to Use
 ----------
 
@@ -85,7 +88,8 @@ With optional parameters:
 ```javascript
 $("#Demo").perfectScrollbar({
   wheelSpeed: 20,
-  wheelPropagation: true
+  wheelPropagation: true,
+  minScrollbarLength: 20
 })
 ```