Add context usage docs to README

[elliottsj]

To help with webpack/webpack.js.org#2289, adding a short description + example of using context.

[codecov]

Codecov Report

Merging #67 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #67   +/-   ##
======================================
  Coverage    91.9%   91.9%           
======================================
  Files          15      15           
  Lines         519     519           
  Branches       71      71           
======================================
  Hits          477     477           
  Misses         39      39           
  Partials        3       3

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[sokra]

Thanks