Add TOC to Readme.md

Thank you for creating such complete documentation for iscrollview!  This is so much better than usual these days.  However there is so much of it I had a hard time keeping track of where I was or whether I had read what I needed to.  So I added a TOC.  I also took the liberty of re-ordering some of the subsections into top-level sections --- hopefully I didn't ruin anything by doing so.