Document get_secret in the readme docs

[SamVanhoutte]

Add the following to the readme + docs

    def get_secret(self, secret_name: str) -> str:
        '''
        Reads a secret string from the registered Azure KeyVault
        Args:
            secret_name (str): the name of the secret key to be registered in Azure KeyVault
        Returns: 
            str: the secret value in the KeyVault
        '''
        keyvault = self.__workspace.get_default_keyvault()
        try:
            return keyvault.get_secret(name=secret_name)
        except:
            return None