Added "ruby-gem-downloads-badge" as a project that uses shields

ruby-gem-downloads-badge is a project that shows the downloads count of a gem , but you can also specify a version or just "stable" and will display the download count of the latest stable version , or it can also display the total download count. 
Here's some examples : 
 ```
  ![](http://ruby-gem-downloads-badge.herokuapp.com/rails)
```
 ![](http://ruby-gem-downloads-badge.herokuapp.com/rails)

```
  ![](http://ruby-gem-downloads-badge.herokuapp.com/rails?type=total)
```

 ![](http://ruby-gem-downloads-badge.herokuapp.com/rails?type=total)

You can find more details on the readme file of the repository : https://github.com/bogdanRada/ruby-gem-downloads-badge