chore: README update (#5)

* chore: README update

It wasn't obvious to me that I needed to install submodules; adding a
quick note on the commands required to get started with this example.

* Update README.md

---------

Co-authored-by: Noah Pendleton <[email protected]>

Author: topher200

README.md

@@ -5,6 +5,17 @@ here relative to the ESP-IDF SDK root folder:
 
 - `examples/system/console/advanced/`
 
+```bash
+$ git submodule update --init --recursive
+$ echo 'CONFIG_MEMFAULT_PROJECT_KEY="YOUR_KEY"' >> sdkconfig.defaults
+# by default, 'idf.py build' will target esp32
+$ idf.py build
+# flash the board and start the serial monitor
+$ idf.py flash monitor
+```
+
+Get your project key from https://mflt.io/project-key.
+
 ## Configuring for MQTT
 
 This application includes an option to send Memfault data over MQTT. This option requires a few extra pieces to set up.