update docs (#764)

hack/docs/Adding_Detectors_Internal.md

@@ -123,3 +123,7 @@ Do not embed test credentials in the test code. Instead, use GCP Secrets Manager
    ```bash
    make protos
    ```
+### Testing a detector
+```bash
+   go test ./pkg/detectors/<detector> -tags=detectors
+   ```

hack/docs/Adding_Detectors_external.md

@@ -100,3 +100,8 @@ Now, the detector test should attempt to load the given env key from that file.
    ```bash
    make protos
    ```
+
+### Testing a detector
+```bash
+   go test ./pkg/detectors/<detector> -tags=detectors
+   ```