Add .npmignore about camshaft HOT 1 CLOSED

$ npm pack camshaft
camshaft-0.61.11.tgz

$ tar -xzf camshaft-0.61.11.tgz

$ ncdu package

  968.0 KiB [##########] /reference                                                                                                                                                                                                                                                       
  528.0 KiB [#####     ] /test
  448.0 KiB [####      ] /lib
  136.0 KiB [#         ] /examples
   20.0 KiB [          ] /docs
   20.0 KiB [          ] /config
   20.0 KiB [          ] /tools
   16.0 KiB [          ]  CHANGELOG.md
    8.0 KiB [          ]  .jshintrc
    8.0 KiB [          ] /.github
    4.0 KiB [          ]  LICENSE
    4.0 KiB [          ]  HOWTORELEASE.md
    4.0 KiB [          ]  .travis.yml
    4.0 KiB [          ]  package.json
    4.0 KiB [          ]  Makefile
    4.0 KiB [          ]  README.md
    4.0 KiB [          ]  CONTRIBUTING.md
    4.0 KiB [          ]  .npmignore