add new link

Author: zingale

README.md

@@ -51,8 +51,9 @@ The following are things hopefully will eventually get written:
 
 A PDF version of these notes is available at:
 
-http://bender.astro.sunysb.edu/hydro_by_example/CompHydroTutorial.pdf
+http://open-astrophysics-bookshelf.github.io/numerical_exercises/
 
+(this PDF is automatically generated by a Github action each time changes are pushed to `main`.)
 
 There are two sets of companion codes that go along with these notes: