Features, Installation and updates, Configuration and typos

Streamline descriptions and remove some lists of specific features. Also
reorder headings to focus on features with lists and pretty pictures to
make things more attractive.

I felt that drawing attention to specific Emacs packages in the readme
would be meaningless to people coming from Vim. They're covered in the
main documentation file anyway.

Properly define what a configuration layer is, and describe the main
roles of the ~/.spacemacs file.

Provide a brief overview on how to load config layers, since this is the
something users will probably want to do immediately.

Fix typos: Identified in review of c36a36fecf7