doc/faq.md: add section on changing keys
doc/faq.md: add section on changing keys

Written mostly be Eric Grosse, edited by me.

Change-Id: I847702d34691a5684ba595387ca597068ab18221
Reviewed-on: https://upspin-review.googlesource.com/12180
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>

upspin/upspin,upspin/upspin