Add Sentry env-var to README

Author: mostafa

README.md

@@ -29,3 +29,7 @@ make build-dev
 ```
 
 Running the above command causes the `go mod tidy` and `go build` to run for compiling and generating the plugin binary in the current directory, named `gatewayd-plugin-cache`.
+
+## Sentry
+
+This plugin uses [Sentry](https://sentry.io) for error tracking. Sentry can be configured using the `SENTRY_DSN` environment variable. If `SENTRY_DSN` is not set, Sentry will not be used.