Add matrix-synapse-shared-secret-auth as an example password provider (#7248)

changelog.d/7248.doc

+Add documentation to the `password_providers` config option. Add known password provider implementations to docs.
\ No newline at end of file

docs/password_auth_providers.md

@@ -13,6 +13,7 @@ own password auth providers. Additionally, here is a list of known
 password auth provider module implementations:
 
 * [matrix-synapse-ldap3](https://github.com/matrix-org/matrix-synapse-ldap3/)
+* [matrix-synapse-shared-secret-auth](https://github.com/devture/matrix-synapse-shared-secret-auth)
 
 ## Required methods