DOC: remove references to CPU=native in the README

Certain compilers running in virtualized environments may produce code
that the same processor cannot execute with -march=native, either because
of hypervisor bugs reporting wrong CPU features, or because of compiler
bugs forgetting to check CPU features. So better stop recommending this
combination so that users don't get trapped anymore.
(cherry picked from commit 817dad50b02d1a82d495dfea4eab9e3a91127391)