Add profiling instructions to CONTRIBUTING

CONTRIBUTING.md

@@ -69,6 +69,29 @@ npm run lint
 npm run format
 ```
 
+## Profiling
+
+If you need to do some performance profiling you can run the debug server with
+`FLASK_PROFILE=True`. You’ll need to first create a directory for the profile
+files:
+
+```
+mkdir -p tmp/profiler
+FLASK_PROFILE=True poetry run flask run --debug
+```
+
+There are multiple ways you can view the `.prof` files generated by the
+profiling middleware. One easy way to view them is with snakeviz.
+
+```
+poetry run pip install snakeviz
+poetry run snakeviz tmp/profiler/<endpoint>.prof
+```
+
+That first line installs snakeviz in your virtual env (without adding it as a
+dependency to the project) and the second will load a performance profile into
+your default browser for you to view.
+
 ## Coding Style
 
 ### Python