MPL-30 Add gitter chat badge (#30)

A simple way to contact developers and discuss the issues

Author: Sergei Parshev

README.md

@@ -1,7 +1,7 @@
 Modular Pipeline Library
 ========================
 
-[![CircleCI](https://circleci.com/gh/griddynamics/mpl/tree/master.svg?style=shield)](https://circleci.com/gh/griddynamics/mpl)
+[![CircleCI](https://circleci.com/gh/griddynamics/mpl/tree/master.svg?style=shield)](https://circleci.com/gh/griddynamics/mpl) [![Gitter](https://badges.gitter.im/modular-pipeline-library/community.svg)](https://gitter.im/modular-pipeline-library/community)
 
 [![CircleCI nightly LTS](https://img.shields.io/badge/dynamic/json?label=nightly%20lts&query=%24%5B%3F%28%40.branch%20%3D%3D%20%22master%22%20%26%26%20%40.workflows.workflow_name%20%3D%3D%20%22nightly_jenkins_test%22%20%26%26%20%40.workflows.%20job_name%20%3D%3D%20%22jenkins_test-1%22%29%5D.status&url=https%3A%2F%2Fcircleci.com%2Fapi%2Fv1.1%2Fproject%2Fgh%2Fgriddynamics%2Fmpl%3Flimit%3D10)](https://circleci.com/gh/griddynamics/workflows/mpl/tree/master) - testing MPL pipeline with the current LTS Jenkins every night