clarify CI-based error detection

docs/book/README.md

@@ -14,4 +14,4 @@ This will open the book it in your default browser and set up a local mdbook ser
 
 In order to include source code in the book, create an accompanying Python file for your section. Then, include sections of the Python file in the markdown using [mdbook partial file inclusion features](https://rust-lang.github.io/mdBook/format/mdbook.html#including-portions-of-a-file).
 
-The Python file should be able to run on its own and must be tested by CI. Prefer importing from adjacent Python files instead of redefining when your section depends on previous sections' definitions.
+The Python file should be able to run on its own and must be tested by CI (any Python file added to this directory will be ran by CI for errors). Prefer importing from adjacent Python files instead of redefining when your section depends on previous sections' definitions.