README.txt: Rewrite to Markdown syntax

I thought you might consider adapting your Readme to a format that Github can parse. This way you have more power over how your Readme is presented to the public. Have a look [at my fork](https://github.com/erikb85/java-simple-serial-connector), if you want to see how it looks. It's the same syntax used for comments *like this one* by the way.

Of course changes to this pull request are still possible.