docs(readme): Document createUnsignedAssertion() and improve usage examples

[esarafianou]

Description

1. saml20.createUnsignedAssertion() and saml11.createUnsignedAssertion() were added in version 0.15 but are not documented in README
2. All exported functions work with callback, but the usage examples have them as sync functions. This PR fixes this

References

• Changelog for 0.15: https://github.com/auth0/node-saml/blob/v1.0.0/CHANGELOG.md#0150-2020-10-01

Testing

Only docs changes

[luuuis]

Synchronous invocation is actually supported unless the encryptionCert option has been passed in, at which point callback becomes required. It's a bit silly but that's how the library has behaved.

node-saml/lib/saml20.js

Lines 261 to 263 in 06acc02

	if (strategies.encryptXml === EncryptXml.unencrypted) {
	return strategies.encryptXml(signed, callback);
	}

I'm ok with the change though.