Add mattermost to Trusted By (#863)

Author: agnivade

README.md

@@ -138,6 +138,7 @@ The following companies/products use golangci-lint:
 * [The New York Times](https://github.com/NYTimes/encoding-wrapper)
 * [Istio](https://github.com/istio/istio)
 * [SoundCloud](https://github.com/soundcloud/periskop)
+* [Mattermost](https://github.com/mattermost/mattermost-server)
 
 The following great projects use golangci-lint:
 

README.tmpl.md

@@ -138,6 +138,7 @@ The following companies/products use golangci-lint:
 * [The New York Times](https://github.com/NYTimes/encoding-wrapper)
 * [Istio](https://github.com/istio/istio)
 * [SoundCloud](https://github.com/soundcloud/periskop)
+* [Mattermost](https://github.com/mattermost/mattermost-server)
 
 The following great projects use golangci-lint: