[DOC] Plugin_Info for getkirby-plugins.com

PLUGIN_INFO.md

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+#### Installation
+store this file in
+
+	site/plugins/
+
+#### Usage
+use it in your templates the same way as the "normal" thumb function:
+
+    ThumbExt($pic, array('width' => 200, 'srcset' => '2x, 3x'))
+
+the attributes are the same as the standard thumb function, but I´ve added:
+
+- srcset: string (with the pixel density descriptors, seperated by commas)  
+if you want 1x only, simply type `'1x'`. But 1x is always added, so you don´t have to type `'1x, 2x'`, you can leave 1x out.
+- inline-size: boolean (default true)  
+you can turn off the inline size feature of this plugin
+- srcset-only: boolean (default false)  
+this enables that there is only the srcset-string returned, e.g. for [picturefill](http://scottjehl.github.io/picturefill/).
+
+The standard function of this plugin is, that this returns a full img-tag ready to use in your template or snippet.  
+By default 1x and 2x images are generated, which is the most often used case. But you can define any pixel ratio you want to.
+
+## contribution
+Feel free to fork this repository an make it better.  
+Perhaps we can implement the viewport width feature in some way.