Some README Tweaks

A few quick tweaks to the README.md while I started following the
instructions.
Various improvements in the README
Merge "Previewing" with "Dependencies"
Move "Prevew Text Changes" under "Translation"
Revert one broken url (Updating English strings)
Shorten instructions as much as possible
Add missing / incomplete instructions for jekyll server
Add missing "jekyll build" / "jekyll serve" commands
Capitalize all titles consistently