README: Add a mini gif tutorial of a real project

[kataras]

I assert that this patch is my own work, and to simplify the licensing of the BFG Repo-Cleaner:

• I assign the copyright on this contribution to Roberto Tyley
• I disclaim copyright and thus place this contribution in the public domain

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Hello @rtyley, I've just found about this software. I have used it to clean the git's size of my 18k starred project: https://github.com/kataras/iris. So, I decided to create a mini gif "tutorial" for newcomers, this is a real-world example of usage of your software. Again, congrats for this one!

P.S: you can edit the commit, position the gif whenever you want, or even remove it and place it on the github page(https://rtyley.github.io/bfg-repo-cleaner/) instead, it's your choice.

Thanks,
Gerasimos Maropoulos