doc: Update readme to include link to reference implementation.

[bjorndown]

I found this information too hard to find.

[markt-asf]

There is no such thing as a reference implementation. There are compatible implementations which depend on version. It isn't always Expressly (although I expect Expressly with pass the 6.0 TCK soon).

Compatible implementations can be found via https://jakarta.ee/specifications/expression-language/ (each version has its own set of CIs).

I think an update to the readme to link to https://jakarta.ee/specifications/expression-language/ would be reasonable.

[bjorndown]

There is no such thing as a reference implementation

Oh ok that is interesting because here it says reference implementation: https://docs.jboss.org/hibernate/stable/validator/reference/en-US/html_single/#validator-gettingstarted-uel

But I noticed that they link to https://github.com/eclipse-ee4j/el-ri which redirects to this repo here. I guess somebody only updated the maven dependency instead of rephrasing the paragraph.

I am fine with linking to https://jakarta.ee/specifications/expression-language/ because that makes it easier for people to find an implementation.

[markt-asf]

Replaced with #249