add notes about week 2 tools

Signed-off-by: Alex Chi <[email protected]>

Author: skyzh

book/src/setup.md

@@ -75,4 +75,6 @@ pdm run python main_ref_impl_week1.py
 
 It should load the model and print some text.
 
+In week 2, we will write some kernels in C++/Metal, and we will need to set up additional tools for that. We will cover it later.
+
 {{#include copyright.md}}

build_ext.sh

@@ -1,4 +1,4 @@
 #!/bin/bash
 
 cd src/extensions_ref
-python setup.py build_ext --inplace
+pdm run python setup.py build_ext --inplace