Choose or create a favicon for helloSystem

Spun off partly from https://github.com/helloSystem/hello/issues/42

If (eventually) there'll be a non-GitHub _home_ page (or _forum_ or whatever) for helloSystem, then now's a good a time as any to think about a favicon.

At the time of writing, I think that the cursive _hello_ is fairly recognisable, however I can't guess whether (for example) the _h_ alone would lend itself to a favicon.

(That's not to suggest that 'h' is the way to go. Just kicking the ball around.)

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