Convert README.md to markdown

[tjunier]

Thanks! I like the idea of having a Markdown version of the README, but I want to keep the plain-text one as well, so I will keep both.

[abitrolly]

@tjunier I've edited PR to include markdown file without the removal of original.

[corneliusroemer]

Would be great to add links to the doc pdfs as they are a bit hidden among hundreds of files, see #33

• https://github.com/tjunier/newick_utils/blob/master/doc/nwutils_tutorial.pdf
• https://github.com/tjunier/newick_utils/blob/master/doc/manual.pdf