Write README in markdown

Otherwise, it won't render nicely on github. In addition, I also added
some markup and commands to run.

Please note that if you don't explicitly type out the commands for
people to run, careless people like me will miss reading it in the
README and you'll spend more time serving support requests. :)
Remove old Known Issue

It's resolved according to @lukipuki, see:

https://github.com/Tarrasch/problemtools/commit/b1e6641a80f3f42a26a15c67b385d39d591e5e1b#commitcomment-8588216
Merge pull request #5 from Tarrasch/rewrite-readme

I agree that not merging this makes no sense, so I'm not waiting for input from anyone else.