Update readme to acknowledge that Entity Reference is now in core

[stpaultim]

Is there any future for this module? Should we update the README to indicate that Entity Reference is now in core?

@jromine

[olafgrabienski]

Is there any future for this module?

I agree, some indication to Entity Reference would be helpful, also if Entity Reference provides already the same features, e.g. regarding integration with other modules. And provide information if it's possible to switch to Entity Reference, or not yet.

[herbdool]

There's a PR for this #20. @jromine. This may require the @backdrop-contrib/bug-squad

[olafgrabienski]

There's a PR for this #20.

Ah, good to know there is https://github.com/backdrop-contrib/entityreference_migration, and the PR mentions it.