Add note about extra build requirements.

Author: dnwpark

gel/_internal/arch.md

@@ -148,6 +148,29 @@ Run `python tools/gen_models.py` to generate test models into your virtual envir
      and you should be able to just run `$ pytest`.
 
 
+### Setup gotchas
+
+#### Using a system-wide `pytest` install
+
+Errors like `ModuleNotFoundError: No module named typing_inspection’` can
+occur when using a system wide pytest installation.
+
+This can be caused by having run either `pip install pytest` outside a venv
+or `apt install python3-pytest`.
+
+If you want to keep these installations, you can still run tests locally by
+running `python -m pytest`.
+
+#### `make clean && make` to fix binary incompatibilities
+
+Errors such as:
+```bash
+gel.protocol.protocol.ExecuteContext size changed, may indicate binary
+incompatibility. Expected 152 from C header, got 184 from PyObject
+```
+
+Indicate something has changed a `pyx` file. A clean rebuild is necessary.
+
 ### Running Tests
 
 ```bash