Add note reminding to pull changes to requirements.txt into inner-sou…

…rce (Qiskit#1321)

The inner-source repo uses this requirements file too, but we pin to a
commit to avoid unintentionally breaking CI. This note reminds
maintainers to update the commit in the inner-source repo after they
update this file.

scripts/nb-tester/requirements.txt

@@ -1,5 +1,7 @@
 # We pin to exact versions for a more reproducible and
-# stable build.
+# stable build. Make sure to update the commit hash in
+# the inner-source repo after updating this file.
+
 qiskit[all]~=1.0
 qiskit-aer~=0.14.0.1
 qiskit-ibm-runtime~=0.23.0