Add XKCD cartoon to readme

[jjb]

as proposed in #32

see preview here: https://github.com/jjb/explainshell/tree/patch-1

(this is the last time I'll bring this up, I promise)

[Artoria2e5]

Wow such idea.

[akshay2000]

I think there should be a space after the # for proper markdown parsing.

[jjb]

@akshay2000 what do you mean? there is no # in my PR

[akshay2000]

I don't know. This is how it rendered for me.

[jjb]

@akshay2000 gotcha -- yeah that was from the code i branched off of. i rebased now, it's fixed.