grpc/doc/ssl-roots-refresh.md

The etc/roots.pem file is generated using:

1. The root certificates from Mozilla: https://hg.mozilla.org/mozilla-central/raw-file/tip/security/nss/lib/ckfw/builtins/certdata.txt

2. A tool to convert them to PEM format: https://github.com/agl/extract-nss-root-certs.