Add some basic usage examples

[marcmo]

Luke your library is awesome and has been pretty useful to me on numerous occasions.
Still what I think is really missing is a small section that shows the usage of your combinators, e.g.

• how to use the memoSecond/memoThird?
• memo2/memo3
• ...

I'm sure this seems obvious to you but also that your library would greatly benefit from having a little more help on those things...

If you feel this is not necessary go ahead and close this again...just wanted to bring this to your attention from a user ;)

[bgwines]

hi, there! Is this going to get some attention? People are clearly confused about how to use this library; some better documentation would be super super helpful:

• https://stackoverflow.com/questions/30319423/haskell-data-memocombinators-performance-issues
• https://stackoverflow.com/questions/38665083/data-memocombinators-not-catching-cases-in-presence-of-extra-variable

thanks! :)