Document in README why =/+ key is not mirrored

[extemporalgenome]

I noticed from the README diagrams that the `/~ key is mirrored repeated twice when mirrored, and the =/+ key isn't mirrored at all. I imagine there's a specific reason for this tradeoff. It'd be be useful for a brief explanation to be included in the README.

Thanks!

[ZevEisenberg]

Good catch! That key isn't mirrored on the Matias Half keyboard either, and it looks like - and + require entering a mode to type special symbols. From the Matias user manual, page 22/23:

But Keyb does not have such a mode. I'd love if someone would add one! It's unfortunately not high on my priority list right now, but perhaps some day.