Formatted readme as markdown

Hi and thanks for maintaining a great tool!

I've found the plaintext readme is hard to read, so I reformatted it in Markdown. The diff is gigantic but I haven't changed any of the content whatsoever. I've also refrained from using the fancier features of Markdown to keep it readable for those who do not view it on Github or through a parser.

I think that this makes it a lot more pleasant to read on Github without making it less pleasant for those who don't.

**Changes:**

- replaced `=`-headers with `#`-headers
- put quotes after angle brackets `>` 
- bumped code-blocks in a step to be picked up as code (still 2 space indentation within the code blocks though)
- replaced `+` notation with markdown code-ticks
- put a couple of things in lists to keep them underneath each other
- put the "Known Extensions" within code-fences to not screw up the formatting, it could be made as a table as well but it's a little trickier. If you're planning to accept this PR and want it as a table just let me know.

Cheers!