docs: BK plugin env vars are named after the plugin

src/plugin/config.go

@@ -11,6 +11,7 @@ type Config struct {
 type EnvironmentConfigFetcher struct {
 }
 
+// TODO: Replace "EXAMPLE_GO" with the name of your plugin, also in upper case
 const pluginEnvironmentPrefix = "BUILDKITE_PLUGIN_EXAMPLE_GO"
 
 func (f EnvironmentConfigFetcher) Fetch(config *Config) error {