[TASK] Add a README that describes the update steps

README.md

+## How to update the sources?
+
+1. Download the latest version from "http://www.tinymce.com/download/download.php"
+2. Download the latest language pack from "http://www.tinymce.com/i18n/index.php".
+   You can simplify the selection by using the following command in your developer tools console.
+    
+    $('input[type=checkbox]').attr('checked', 1)
+
+3. Unpack the downloaded sources
+4. Remove the tinymce directory inside the extension and move "tinymce/js/tinymce" from the download source as a
+   replacement.
+5. Move the files of the "langs" directory from the downloaded languages into "tinymce/langs/"
+6. Update the VERSIONS.md file
+7. Push the new code to git (check the differences and test the stuff before!)
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