Add pseudo-secure key generation to README

Secure key generation is a topic of research. We can provide pseudo-secure methods in the README to help developers get started at least:

- https://www.openssl.org/docs/man1.0.2/man1/openssl-rand.html
- https://stackoverflow.com/questions/40277918/shell-script-to-generate-random-hex-numbers

We can also add resources for folks to learn more about it.