Add comment pointing to commit message that explains why we need to pin

setuptools to a specific version for now.

circle.yml

@@ -4,6 +4,8 @@ machine:
 dependencies:
   override:
     - "pip install -U pip wheel"
+    # Temporarily pin setuptools to a specific version.
+    # See commit message of https://github.com/open-craft/problem-builder/commit/51277a34fb426724616618c1afdb893ab2de4c6b for more info:
     - "pip install setuptools==24.3.1"
     - "pip install -e git://github.com/edx/xblock-sdk.git@bddf9f4a2c6e4df28a411c8f632cc2250170ae9d#egg=xblock-sdk"
     - "pip install -r requirements.txt"