Fixed typos and grammar. This fixes reiven/tomber#2

Fixed a few typos and improved the readability of the file. I went
ahead and rephrased a few portions of the file as well, for example, I
rewrote the part that mentioned the pyTomb code, which no longer is
supported.
made minor edits
Merge pull request #3 from JoelMon/typo-fix

Fixed grammar and typos on README.mkdn