Add readme and screenshot

Author: markjszy

README.md

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+# SimpleSelectionExport
+
+SimpleSelectionExport is a [KeePass2](https://keepass.info) plugin which allows you to export basic entry 
+information to flat text files. It's useful when you want to send a quick overview of an arbitrary set of 
+entries -- selected from the main window -- to parties who are not using KeePass.
+
+![Screenshot](screenshot.png)