add read to automatic derived operations for triples

[andrewufrank]

In an application I produce and read triples (mostly for error tracking) and found that Triple is automatically instantiated for Show but not for Read. Is this intentional?
Changing the code to include Read seems to work (at least I could compile and my code does not show any problem). I would appreciate if this change could be incorporated in the hackage version.
thank you!
andrew