[Checkers] Changing @Romaybe to @PolyRead

[Michael Ernst]

To follow up with a conversation that I had with Jaime, there is consensus
that we should change the @Romaybe keyword to @PolyRead.

I've asked Jaime and Telmo to make this change to their codebases,
effective immediately.  That way, it will appear in the Javarifier release
that Jaime plans to make around Sunday/Monday, and in the Checkers
Framework development release that I presume will be made today.

                     Thanks,

                    -Mike