diff --git a/README.md b/README.md
index 0efc4cf85..ac2d14eac 100644
--- a/README.md
+++ b/README.md
@@ -86,6 +86,8 @@ if [ ! -e /usr/include ]; then sudo ln -s `xcrun --show-sdk-path`/usr/include /u
 pyenv install 2.7.11
 pyenv virtualenv 2.7.11 quay
 pyenv activate quay
+
+# Some packages may fail to build with clang (which now defaults to C11).
 CFLAGS='-std=c99' pip install -r requirements.txt
 pip install -r requirements-dev.txt