Add more about how to *counter* quantum computers

[david-a-wheeler]

Make it clear that if you are adding post-quantum crypto, add it as new layer since there's a non-trivial risk of complete breakage. Include a note about SIKE, where this has already occurred.

[david-a-wheeler]

I got an out-of-band response from Hart Montgomery that this looks good. We can always improve this further, but telling people these basics now seems like the right thing to do.