docs: add Entrust Datacard to README ZLInt users. (#451)

Per Bugzilla[0] Entrust reports using ZLint:

> we are using both pre-issuance linting and post-issuance linting using
> zlint.

[0]: https://bugzilla.mozilla.org/show_bug.cgi?id=1648472

README.md

@@ -159,6 +159,7 @@ Here are some projects/CAs known to integrate with ZLint in some fashion:
 * [Sectigo](https://sectigo.com/) and [crt.sh](https://crt.sh)
 * [Digicert](https://www.digicert.com/)
 * [EJBCA](https://download.primekey.com/docs/EJBCA-Enterprise/6_11_1/adminguide.html#Post%20Processing%20Validators%20(Pre-Certificate%20or%20Certificate%20Validation))
+* [Entrust Datacard](https://www.entrust.com/)
 * [Google Trust Services](https://pki.goog/)
 * [Government of Spain, FNMT](http://www.fnmt.es/)
 * [Globalsign](https://www.globalsign.com/en/)