doc: add debug info

DEBUG.md

@@ -0,0 +1,16 @@
+# Debug
+
+The recommended way to debug a packer plugin is adding prints and running with logs enabled.
+As written in [packer debug documentation](https://developer.hashicorp.com/packer/docs/debugging), you can use `PACKER_LOG=1` to enable verbose logging.
+
+## Testing local build
+
+To test a local build, you need to have the plugin in the same folder where you will run packer:
+
+
+```
+> ls
+packer-plugin-scaleway build_scaleway.pkr.hcl
+> PACKER_LOG=DEBUG SCW_DEBUG=1 packer build build_scaleway.pkr.hcl
+...
+```