Add how to install rack-test from github to README.

[junaruga]

We are preparing next version release for rack-test.
However users may want to use unreleased version now before that.
In that case, it is useful for users to know the way to install unreleased version.

You can see modified README here.
https://github.com/junaruga/rack-test/blob/feature/readme-install-from-github/README.md

[junaruga]

I changed "rack/test" to 'rack/test'. This modification is because rubocop is valid for the single quote code, and it looks more popular.

[junaruga]

I tested like this.

$ cat Gemfile
source 'https://rubygems.org'

# gem 'rack-test', require: 'rack/test'
# gem 'rack-test', github: 'rack-test', branch: 'master'
# gem 'rack-test', github: 'rack-test', ref: 'a5aa1e5eac12bc8007a4bc6af101c24bdc4a2b2e'
gem 'rack-test', github: 'rack-test', ref: 'a5aa1e5'

$ bundle install --path vendor/bundle

$ bundle list
Gems included by the bundle:
  * bundler (1.14.6)
  * rack (2.0.3)
  * rack-test (0.6.3 a5aa1e5)

And related document of Bundler is here. http://bundler.io/v1.15/git.html

[junaruga]

This modification is that I wanted to show same color for the section of Install as a gem and Install via Bundler.
<code>..</code>, <pre>...</pre> are displayed as different color on github markdown.

[perlun]

I think we could skip the second line. Most users will probably not go with a specific ref anyway, but rather a named branch.

[dennissivia]

I agree with that.

[junaruga]

ok I will remove the line.

[junaruga]

Rebased!

[perlun]

Looks good now, thanks!

[perlun]

(rebase again, then feel free to merge)

[junaruga]

Thanks! Merged.