add `venv` instructions to `privategpt` example

examples/privategpt/README.md

@@ -4,6 +4,13 @@
 
 ### Setup
 
+Optionally set up a virtual environment:
+
+```
+python3 -m venv .venv
+source .venv/bin/activate
+```
+
 Install the Python dependencies:
 
 ```shell