Suggestion: Change sha512 to yescrypt in Readme.md

[cweickhmann]

Suggestion to change any occurence of sha512 as hash function for pam_unix in the examples to yescrypt.

Reason: Yescrypt superseeded sha512 for password hashing in most Linux distros as it is more resilient to password-cracking attempts (see e.g. Wikipedia). Examples should have security in mind as default to make noob-mishaps less likely.