Add Gitter badge (#20)

Author: gitter-badger

README.md

@@ -6,7 +6,7 @@ Php-codec is a partial porting of [io-ts](https://github.com/gcanti/io-ts) in PH
 
 [![CI](https://github.com/facile-it/php-codec/actions/workflows/ci.yaml/badge.svg?branch=master&event=push)](https://github.com/facile-it/php-codec/actions/workflows/ci.yaml)
 [![Static analysis](https://github.com/facile-it/php-codec/actions/workflows/static-analysis.yaml/badge.svg?branch=master&event=push)](https://github.com/facile-it/php-codec/actions/workflows/static-analysis.yaml)
-[![codecov](https://codecov.io/gh/facile-it/php-codec/branch/master/graph/badge.svg?token=HP4OFEEPY6)](https://codecov.io/gh/facile-it/php-codec)
+[![codecov](https://codecov.io/gh/facile-it/php-codec/branch/master/graph/badge.svg?token=HP4OFEEPY6)](https://codecov.io/gh/facile-it/php-codec) [![Join the chat at https://gitter.im/php-codec/community](https://badges.gitter.im/php-codec/community.svg)](https://gitter.im/php-codec/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
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