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.
Coffeescript preferred styles

* Use `@` instead of `this` for scoping consistency
* Prefer double quotes for consistency
Clarify SRP guideline

The single responsibility principle sounds more appropriate when
referring to a class as a whole rather than an object.
Don't use spaces after unary operators

The current guide suggests spaces around all operators, but we typically don't
add spaces after unary operators, such as `!`.

Adopted from #163.
Prefer `cookies.signed` in Rails

`cookies` is fine for storing non-sensitive data, but using `cookies.signed`
makes sense as a default. Using `cookies.signed` means you don't need to worry
about whether data is sensitive.

Cookies, even signed ones, can be easily read by the end-user (see
http://www.andylindeman.com/2013/02/18/decoding-rails-session-cookies.html).
Using `cookies.signed` only prevents the end-user from *writing* them.
Use v#.#.# convention for OSS release commits

This keeps us in line with tools such as
https://github.com/tpope/git-bump and lets us use tools to auto-promote
releases when we have a consistent interface.
Raise instead of returning false in AR callbacks

We never check whether a `#destroy` fails in our controllers:

    def destroy
      article = Article.find(params[:id])
      article.destroy
      redirect_to articles_url
    end

Because of this history, it would cause confusion and potential bugs if
we introduced a normal `before_destroy` into any of our models. Not only
will we need to check the result of `#destroy` for any controller using
that model, but we'd also need to check the return value for any
controller using models that associate with that model.

Since callbacks are used for validated database objects and are expected
to succeed, we should raise on failure. This will roll back the
transaction and notify the exception handler (e.g.  Airbrake).

This is true of all callbacks, not just `before_destroy`.
Fix link for "Bundler binstubs"
Prefer `protected` over `private` for non-public attributes

- With warnings enabled, private attributes cause a warning on MRI
  (doubly important for open source libaries, which should not emit
  warnings)
- Reduces churn when adding comparison or equality methods, since you'll
  likely need to access the attributes of the other object
- Reduces churn when adding a writer, since it is impossible to call
  private writer methods.
Specify guideline for required keyword arguments
Prefer `{Date,Time}.current` over `.today`/`.now`

`Date.current` and `Time.current` take `Time.zone` and
`config.time_zone` into account, falling back to `Date.today` and
`Time.now` respectively if a zone isn't set.
Use double quotes in Sass
Add note about allowing for feedback

* We've had several pull requests merged quickly
* Our contribution guidelines request a week to allow feedback