Add installation instructions to the README

On copying the documentation from heroku I came accross installation instructions that seem to have gotten lost when the project came to github. I felt it would be useful to reintegrate them in the README. I don't know if there was a motivation for not bringing them in from heroku.

On copying the documentation from heroku I came accross installation instructions that seem to have gotten lost when the project came to github. I felt it would be useful to reintegrate them in the README. I don't know if there was a motivation for not bringing them in from heroku.

On running these instructions I had several issues.

```bash
prompt$ gem build *.gemspec
WARNING:  licenses is empty
  Successfully built RubyGem
  Name: rush
  Version: 0.6.8
  File: rush-0.6.8.gem
--- prompt$ sudo gem install rush*.gem                                          
[sudo] password for xxx: 
Fetching: coolline-0.4.3.gem (100%)
Successfully installed coolline-0.4.3
Fetching: coderay-1.1.0.gem (100%)
Successfully installed coderay-1.1.0
Successfully installed rush-0.6.8
Parsing documentation for coolline-0.4.3
Installing ri documentation for coolline-0.4.3
invalid options: -SNw2
(invalid options are ignored)
Parsing documentation for coderay-1.1.0
Installing ri documentation for coderay-1.1.0
Parsing documentation for rush-0.6.8
3 gems installed
--- prompt$ sudo gem install --dev rush*.gem
Fetching: git-1.2.6.gem (100%)
Successfully installed git-1.2.6
Fetching: mini_portile-0.5.3.gem (100%)
Successfully installed mini_portile-0.5.3
Fetching: nokogiri-1.6.1.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing rush-0.6.8.gem:
	ERROR: Failed to build gem native extension.

    /usr/bin/ruby2.0 extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h


Gem files will remain installed in /var/lib/gems/2.0.0/gems/nokogiri-1.6.1 for inspection.
Results logged to /var/lib/gems/2.0.0/gems/nokogiri-1.6.1/ext/nokogiri/gem_make.out
```

If I manage to fix the errors and warnings, I'll file other pull requests.