Explain that PYTHONPATH must be updated

Cloning this repository, then building & running scripts, can run into the issue of the `import gradio` call pulling `gradio` in from somewhere else (such as the virtual environment). To address this, users need to tell python where to look in order to use the cloned git repo for `gradio`.

Further details in https://huggingface.slack.com/archives/C02SPHC1KD1/p1731207233633159?thread_ts=1731206608.882719&cid=C02SPHC1KD1 for more

CONTRIBUTING.md

@@ -109,6 +109,9 @@ if __name__ == "__main__":
    demo.launch()
 ```
 
+Make sure your `PYTHONPATH` includes the directory where this repository is cloned, e.g.,
+`export PYTHONPATH="./"`
+
 then run:
 
 ```
@@ -348,5 +351,18 @@ Run `scripts/build_frontend.sh` with the environment variable `NODE_OPTIONS=--ma
 
 ---
 
+In the case of:
+- Unexpected Exceptions being thrown;
+- The following warning:
+`IMPORTANT: You are using gradio version <earlier version>, however version <later version> is available, please upgrade.`
+
+Make sure your `PYTHONPATH` includes the directory where this repository is cloned, e.g.:
+
+```export PYTHONPATH="./"```
+
+This ensures that when `gradio` is imported in a python program, it is this current version from this repository.
+
+---
+
 _Could these guidelines be clearer? Feel free to open a PR to help us facilitate open-source contributions!_