index.html for github pages; added screenshot to README

Author: Bunkerbewohner

README.md

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 Can you explain a hard idea using only the ten hundred most 
 used words? It's not very easy. A tool inspired by the 
 [XKCD comic "Up Goer Five"](https://xkcd.com/1133/).
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+[Try it yourself](https://bunkerbewohner.github.io/js-up-goer-five/)!
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+![Screenshot of the editor](https://raw.githubusercontent.com/Bunkerbewohner/js-up-goer-five/master/screenshot.png)
+