Update FIPS instructions in README.md

Update the instructions to follow the requirements
in the security policy and user guide

PR-URL: https://github.com/nodejs/node/pull/2278
Fixes: https://github.com/nodejs/node/issues/2242


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Rod Vagg <r@va.gg>