added license to readme #588

[cg-cnu]

A small, simple change 😄
Solves #588

[ideasman42]

Should probably mention licenses of 3rd party deps.

eg:

ISC, with 3rd party modules using MIT.

Then the second part can be extended if other libs are added.

[cg-cnu]

If @setpixel is ok with it I can add.
But AFAIK, we usually mention only the repo's license.
At least that's what they do in the js world 😄

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