Add README

[adamgundry]

This should explain the motivation behind optics, basic usage and the differences from lens.

[arybczak]

Perhaps we should move stuff that currently sits in optics/src/Optics.hs into README?

Then it's displayed on both github and hackage.

[adamgundry]

Good plan. I'm still planning to do a pass over the documentation at some point, but time is ever a problem...

[phadej]

Note: optics/src/Optics.hs has haddock tests; and identifiers are links into documentation of that version.

[adamgundry]

I'm now inclined to make the README relatively minimal, with some brief motivation and a link to the Haddocks on Hackage, rather than duplicating content. But it still needs to be written.