Add user guide for secret store

[ckunki]

Until we have a user guide, please see the unit tests.

Usage sample

from exasol.secret_store import Secrets

file = "password_db.sqlite"
secrets = Secrets(file, "my secret password")
key = "my key"
secrets.save(key, "my value").close()
value = secrets.get(key) 

Constraints and special situations

• If file does not exist then SecretStore will create it.
• If password is wrong then SecretStore will throw an exception.
• If file contains key from a session in the past then method secrets.save() will overwrite the value for this key.
• If key is not contained in file then SecretStore return None

Additional tasks

Besides maybe we should add a test trying to get the value of a non-existing key to verify behavior.

This test already did exist:

def test_database_file_created(secrets):
    assert secrets.get("any_key") is None
    assert secrets.db_file.exists()