Create reset_config.md (#933)

* Create reset_config.md

docs/tools/reset_config.md

@@ -0,0 +1,46 @@
+[**UDMI**](../../) / [**Docs**](../) / [**Tools**](./) / [Reset Config](#)
+
+# Send a configuration message to a specific device
+
+The `reset_config` tool can be used to send a configuration message to a specific device.
+
+The `reset_config` command can be invoked from the `udmi` folder in the following way:
+
+```
+bin/reset_config SITE_DIR PROJECT_ID DEVICE_ID [CONFIG_FILE]
+```
+
+For example, if the UDMI site model is in the `path/to/site_model` folder, if the GCP project ID is 
+`my_project` and the device id is `TEST-1`, the command line will look like this:
+
+```
+bin/reset_config path/to/site_model my_project TEST-1
+```
+
+In this case, the `reset_config` tool will use the `generated_config.json` file that the `registrar` 
+tool has created in the `devices/TEST-1/out` folder.
+
+It is also possible to send a specific `config` payload.
+
+In this case, it is necessary to store a file including the `config` payload in the `devices/DEVICE_ID/config` folder.
+
+For example, if a file named `config.json` has been created for this purpose in the `devices/TEST-1/config` folder,
+it will be possible to invoke the `reset_config` tool with the following command:
+
+```
+bin/reset_config path/to/site_model my_project TEST-1 config.json
+```
+
+Finally, it is possible to get `registrar` to send a user defined config instead of a generated config file.
+
+For this purpose, edit the `metadata.json` file to include the following `cloud` configuration object:
+
+```
+  "cloud": {
+    "config": {
+      "static_file": "config.json"
+    }
+  }
+```
+
+and store the `config.json` file in the `devices/DEVICE_ID/config` folder.