Update "Installation" documentation for Sass

This implements the change in hail2u/vim-css3-syntax#9
Readability fixes in README

Fix minor spelling and grammar errors and add/change section titles to
improve readability.
Rephrase HTML/SCSS headings to improve readability
Enable default support for HTML and SCSS

Most anyone who works with CSS will encounter inline CSS in HTML at some
point, so it follows that this should support that by default.
Additionally, SCSS is quite popular at this point and it makes more
sense to support it by default than to require manually enabling it.
Merge branch 'readme-scss-installation' of git://github.com/jonathanpatt/vim-css3-syntax
Merge branch 'add-html-scss-support' of git://github.com/jonathanpatt/vim-css3-syntax

Conflicts:
	README.md