[GHC] #8190: mention --show-options in --help

[GHC]

#8190: mention --show-options in --help
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        Reporter:  carter            |            Owner:  thoughtpolice
            Type:  feature request   |           Status:  new
        Priority:  highest           |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #7843
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Comment (by nh2):

 As somebody who messes around with tools calling GHC, I would appreciate
 this option.

 It would also make it much easier to find options that are not documented!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8190#comment:7>
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