[ creek ] add badges to README.

Author: pythonicrubyist

README.md

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+[![version](https://badge.fury.io/rb/creek.svg)](https://badge.fury.io/rb/creek)
+[![rubygems.org](https://ruby-gem-downloads-badge.herokuapp.com/creek?type=total&total_label=downloads)](https://ruby-gem-downloads-badge.herokuapp.com/creek?type=total&total_label=downloads)
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 # Creek - Stream parser for large Excel (xlsx and xlsm) files.
 
 Creek is a Ruby gem that provides a fast, simple and efficient method of parsing large Excel (xlsx and xlsm) files.