Update README.md for better usage instructions

This PR helps specify installation instructions regarding the ```a``` tag that wraps fluidbox images.

I was getting this error when implementing fluidbox:
```
Resource interpreted as Image but transferred with MIME type text/html: "http://localhost:8000/media".
```
using this code:
```
    %a.{ href: "#" }
      = image_tag("media/example_image.png")
```
I fixed the error by adding the proper image url to the ```a``` tag.