Add: fuubar To Relish Documentation Under Formatters

[jfelchner]

As discussed in https://github.com/rspec/rspec/issues/61#issuecomment-751519097

@pirj can you let me know if this is what you had in mind?

[JonRowe]

I would write this as a scenario (or scenarios) demonstrating 3rd party formatters.

Suggested change

	Feature: fuubar
	[fuubar](https://github.com/thekompanee/fuubar) is an instafailing formatter
	that uses a progress bar instead of a string of letters and dots as feedback.
	It provides visual feedback of how many specs are left to be run as well as
	a rough approximation of how long it will take.
	
	Installation
	--------------------------------------------------------------------------------
	```ruby
	gem install fuubar
	# or in your Gemfile
	gem 'fuubar'
	```
	Usage
	--------------------------------------------------------------------------------
	In order to use fuubar, you have three options.
	### Option 1: Invoke It Manually Via The Command Line
	```bash
	rspec --format Fuubar --color
	```
	### Option 2: Add It To Your Local `.rspec` File
	```text
	# .rspec
	--format Fuubar
	--color
	```
	### Option 3: Add It To Your `spec_helper.rb`
	```ruby
	# spec/spec_helper.rb
	RSpec.configure do |config|
	config.add_formatter 'Fuubar'
	end
	```
	---
	For more information, you can [view the fuubar
	README](https://github.com/thekompanee/fuubar)
	Feature: Configuring 3rd party formatters
	RSpec supports a variety of 3rd party formatters thanks to our awesome community.
	One such example is [`fuubar`](https://github.com/thekompanee/fuubar) a fast failing formatter which approximates your test suite as a progress bar rather than continuous output, providing visual feedback of how many specs are left to run.
	Scenario: Invoking a 3rd party formatter manually from the command line.
	Given the "fuubar" gem is installed
	Given a file named "spec/example_spec.rb" with:
	"""ruby
	RSpec.describe "examples" do
	it "is an example" do
	end
	it "is another example" do
	end
	end
	"""
	When I run `rspec --format Fuubar spec/example_spec.rb`
	Then the output should contain "<fuubar output>"
	Scenario: Invoking a 3rd party formatter via a .rspec file
	Given the "fuubar" gem is installed
	Given a file named ".rspec" with:
	"""
	--format Fuubar
	"""
	And a file named "spec/example_spec.rb" with:
	"""ruby
	RSpec.describe "examples" do
	it "is an example" do
	end
	it "is another example" do
	end
	end
	"""
	When I run `rspec spec/example_spec.rb`
	Then the output should contain "<fuubar output>"
	Scenario: Invoking a 3rd party formatter via your spec_helper.rb
	Given the "fuubar" gem is installed
	Given a file named "spec/spec_helper.rb" with:
	"""ruby
	RSpec.configure do |config|
	config.add_formatter 'Fuubar'
	end
	"""
	And a file named "spec/example_spec.rb" with:
	"""ruby
	RSpec.describe "examples" do
	it "is an example" do
	end
	it "is another example" do
	end
	end
	"""
	When I run `rspec spec/example_spec.rb`
	Then the output should contain "<fuubar output>"

[pirj]

@jfelchner This build fails since it's not just a text file, it's a runnable scenario.
I'll be happy to lend a hand with this if you get stuck with anything.

[jfelchner]

Thanks @pirj I'll have to work on this when I get a few minutes.