Add a link to the matrix-synapse-rest-password-provider. (#8111)

changelog.d/8111.doc

+Link to matrix-synapse-rest-password-provider in the password provider documentation.

docs/password_auth_providers.md

@@ -14,6 +14,7 @@ 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)
+* [matrix-synapse-rest-password-provider](https://github.com/ma1uta/matrix-synapse-rest-password-provider)
 
 ## Required methods