Update README.md

Corrects link to the indentation example on the Haskell section.
Add guideline on symbol to_proc

* Using &:method_name is brief but clear
* Using it consistently aids readability
* Preferring simple method calls encourages simple methods
Avoid `link_to` for non-GET requests.

* Using `link_to` with the `method` option introduces an un-necessary
  dependency on JavaScript.
* HTML elements -- in this case a button or a link -- should be chosen for
  their semantics, not their appearance.
* CSS can be used to make links look like buttons and buttons look like
  links.
Add js- prefix to classes as well as ids that use javascript
Remove guideline to prepend js- to ids using javascript
Link to Liftoff

Move Liftoff guideline out of style, which is intended to be mostly
formatting, and into best practices.