Note that "{HEAD}" is invalid, not special. Fixes: https://github.com/bazelbuild/rules_python/issues/24

diff --git a/README.md b/README.md
index 76791a7..611fee5 100644
--- a/README.md
+++ b/README.md

@@ -24,6 +24,7 @@
 git_repository(
     name = "io_bazel_rules_python",
     remote = "https://github.com/bazelbuild/rules_python.git",
+    # NOT VALID!  Replace this with a Git commit SHA.
     commit = "{HEAD}",
 )