Update readme regarding json config limitations

Author: maxfrei

README.md

@@ -36,6 +36,15 @@ $ packer init examples/basic_example.pkr.hcl
 Installed plugin github.com/upcloudltd/upcloud v1.0.0 in "/Users/johndoe/.packer.d/plugins/github.com/upcloudltd/upcloud/packer-plugin-upcloud_v1.0.0_x5.0_darwin_amd64"
 ```
 
+From Packer version 1.7.0, template HCL2 becomes officially the preferred way to write Packer configuration. While the `json` format is still supported, but certain new features, such as `packer init` works only in newer HCL2 format.
+If you are using `json` config templates, please consider upgrading them using the packer built-in command:
+
+```sh
+$ packer hcl2_upgrade example.json
+Successfully created example.json.pkr.hcl
+```
+
+
 ### Pre-built binaries
 
 You can download the pre-built binaries of the plugin from the [GitHub releases page](https://github.com/UpCloudLtd/packer-plugin-upcloud/releases). Just download the archive for your operating system and architecture, unpack it, and place the binary in the appropriate location, e.g. on Linux `~/.packer.d/plugins`. Make sure the file is executable, then install [Packer](https://www.packer.io/).