create a hyperlink to LICENSE.markdown
.
modest heading levels

“JSZip” becomes H2, any other header becomes H3.

(GitHub's H1 headers are about 37px and that's huge.)
keep “JSZip” on the H1 heading level

(Partial revert of the previous commit.)
continue using H2 for non-topmost headers
Merge pull request #108 from Mithgol/readme-improvements

small README improvements