Add Gerrit OAUTH plugin and setup for GitHub

Using v3.5.1 from
https://github.com/davido/gerrit-oauth-provider/releases/download/v3.5.1/gerrit-oauth-provider.jar
diff --git a/gerrit/environment.template b/gerrit/environment.template
index f8975aa..f0310e0 100644
--- a/gerrit/environment.template
+++ b/gerrit/environment.template
@@ -1,2 +1,6 @@
 # Set to publicly visible Gerrit URL
 # CANONICAL_WEB_URL=https://review.some.host/
+
+# Set to enable GitHub OAUTH
+# GITHUB_OAUTH_CLIENT_ID=
+# GITHUB_OAUTH_CLIENT_SECRET=