Update README.md with proper guidelines for Drupal 8+

Problem/Motivation

The current guidelines in the README.md file are not suitable for Drupal 8+ making this module difficult to use unless you scan its code.

This was somehow addressed as a topic in 📌 How to use the message digest module? Active .

Proposed resolution

Update the README.md file with the new guidelines for Drupal 8+.

Remaining tasks

[ ] Update README.md
[ ] Create a new issue or decide using this one to provide a message_digest_example module as proposed in #2429475-8: How to use the message digest module? → as proposed by @abhaypai.

User interface changes

None

API changes

None

Data model changes

None