Add note and link on short selectors
Add a link pointing to an article on specificity
Add a link to the @extend %placeholder technique
Update section on `@extend`
Fix paragraphy to refer to the correct selector

And clarify the sentence a bit, too.
Update "Nest only when necessary" documentation
Add to table of contents a link to new sub section
Cut out "genuine details" fluff
Remove "caveates" section on extends
Add scss syntax highlights to code examples
Reverse the use of `@extend` in a code example
Merge pull request #36 from mobify/add-note-on-specificity

Post-Documentation Tweaks