Add a Gitter chat badge to README.md

[gitter-badger]

formly-js/angular-formly-templates-material now has a Chat Room on Gitter

@kamilkisiela has just created a chat room. You can visit it here: https://gitter.im/formly-js/angular-formly-templates-material.

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[codecov-io]

Current coverage is 97.43%

Merging #2 into master will not affect coverage as of 947129c

@@            master      #2   diff @@
======================================
  Files           14      14       
  Stmts           39      39       
  Branches         0       0       
  Methods          0       0       
======================================
  Hit             38      38       
  Partial          0       0       
  Missed           1       1       

Review entire Coverage Diff as of 947129c

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