Add note for version 11

[fulldecent]

It appears that version 11 was previously maintained here but it is now no longer maintained.

I am speculating entirely based on the current and past state of files in the repository.

It would be nice if an actual note were added to explain which versions are supported and when they will stop being supported.

[fulldecent]

One motivation for this is reconciling why Travis supports version 11 whereas nobody else seems to.

[nschonni]

The images are still there, but 11 hit EOL at the end of June https://github.com/nodejs/Release#release-schedule