Update readme.md, including a table of contents

Added a table of contents and som cross-referencing anchorlinks plus other minor changes/fixes.
I found it ridiculously hard to scroll all the way down just to find out how the new style settings work and what parameters are accepted, thus I decided to add this table of contents. The list includes all headings level 1 and 2 (h1, h2).
Also, while reviewing the file I changed some minor things like inserting a line break for accepted `icon` parameters because the line was rendered so long that it broke github's layout.
I also changed the last two sections and combined them to "Changelog".