add ET Runner instructions (#691)

Co-authored-by: Michael Gschwind <[email protected]>

README.md

@@ -199,6 +199,22 @@ export TORCHCHAT_ROOT=${PWD}
 ./scripts/install_et.sh
 ```
 
+### Test it out using our Executorch runner
+Build the runner
+```bash
+scripts/build_native.sh et
+```
+
+**Get a PTE file if you don't have one already**
+```
+python3 torchchat.py export llama3 --quantize config/data/mobile.json --output-pte-path llama3.pte
+```
+
+Execute using the runner
+```bash
+cmake-out/et_run llama3.pte -z ~/.torchchat/model-cache/meta-llama/Meta-Llama-3-8B-Instruct/tokenizer.model -i "Once upon a time"
+```
+
 ### Export for mobile
 The following example uses the Llama3 8B Instruct model.