Update Readme; multiple file() needs custom key

- Update the Readme with notes that when using multiple config files,
  the user must use a custom key as the first parameter in additional
  nconf.file() calls in order for config heirarchy/inheritance to work
  properly.

Fixes #97, Fixes #109, Fixes #110
Merge pull request #117 from MitchellMcKenna/master

Update Readme to note additional nconf.file() calls require using a custom key