Update README.md

Author: lostella

README.md

@@ -2,7 +2,6 @@
 
 [![Build status](https://github.com/JuliaFirstOrder/ProximalOperators.jl/workflows/CI/badge.svg?branch=master)](https://github.com/JuliaFirstOrder/ProximalOperators.jl/actions?query=workflow%3ACI+branch%3Amaster)
 [![codecov](https://codecov.io/gh/JuliaFirstOrder/ProximalOperators.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/JuliaFirstOrder/ProximalOperators.jl)
-[![Gitter](https://img.shields.io/gitter/room/nwjs/nw.js.svg)](https://gitter.im/ProximalOperators-jl/Lobby)
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.4020558.svg)](https://doi.org/10.5281/zenodo.4020558)
 
 [![](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliafirstorder.github.io/ProximalOperators.jl/stable)