Add demo .gif of using `rgr` to the readme

[ElectricRCAircraftGuy]

From @abitrolly, here: BurntSushi/ripgrep#74 (comment)

But.. if there was a recorded .gif demo (which fastmod does not) I could at least watch it. Frankly, because of lack of demo, I would ignore fastmod too, but facebook name together with stars convinced my to give it a try and spend 15 minutes to try.

In other words, people think a gif demo would really help promote the utility of this tool. I agree.

Tools I'll need:

1. https://superuser.com/questions/1107200/optimize-animated-gif-size-in-command-line/1107201#1107201
2. https://askubuntu.com/questions/648603/how-to-create-an-animated-gif-from-mp4-video-via-command-line#comment2393212_837574

• Add screenshots too.

[abitrolly]

Listing alternatives is also a good practice to collect stars for the reference.