added coverage badge

Author: adambullmer

README.md

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 DocBlockr for Python
 ====================
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+[![codecov](https://codecov.io/gh/adambullmer/sublime_docblockr_python/branch/master/graph/badge.svg)](https://codecov.io/gh/adambullmer/sublime_docblockr_python)
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 Based off the [DocBlockr](https://github.com/spadgos/sublime-jsdocs) project, This extension provides the similar funtionality but for python docstrings.
 The default formatter for this plugin is designed around [PEP-257](https://www.python.org/dev/peps/pep-0257/) compliance but with more verbosity: Added variable types, listing class extensions, and listing decorators.
 The main goal of this project is to help developer provide better documentation by giving easy and consistent formatting.