markdownified README, added links where appropriate.

Hey guys, first of all thanks for this great software! 
I found a minor typo in the README and then just added some markdown.
Have a look at the result at https://github.com/0x20h/draw.io/tree/markdown-readme

I am still a bit uncertain if I would have to purchase a commercial license for intranet use in a commercial environment (probably due to my english skills). Maybe you can make this more clear a second time?