Add a Gitter chat badge to README.md

[gitter-badger]

gergelypolonkai/word-challenge now has a Chat Room on Gitter

@gergelypolonkai has just created a chat room. You can visit it here: https://gitter.im/gergelypolonkai/word-challenge.

This pull-request adds this badge to your README.md:

If my aim is a little off, please let me know.

Happy chatting.

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[codecov-io]

Current coverage is 100.00%

Merging #2 into master will not affect coverage as of deea623

@@            master      #2   diff @@
======================================
  Files           10      10       
  Stmts          271     271       
  Branches         0       0       
  Methods          0       0       
======================================
  Hit            271     271       
  Partial          0       0       
  Missed           0       0       

Review entire Coverage Diff as of deea623

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