Add note about autovivification
diff --git a/compiler/util/ir_data_utils.py b/compiler/util/ir_data_utils.py
index c15700c..d2f0536 100644
--- a/compiler/util/ir_data_utils.py
+++ b/compiler/util/ir_data_utils.py
@@ -14,6 +14,13 @@
"""Provides a helpers for working with IR data elements.
+Historical note: At one point protocol buffers were used for IR data. The
+codebase still expects the IR data classes to behave similarly, particularly
+with respect to "autovivification" where accessing an undefined field will
+create it temporarily and add it if assigned to. Though, perhaps not fully
+following the Pythonic ethos, we provide this behavior via the `builder` and
+`reader` helpers to remain compatible with the rest of the codebase.
+
builder
-------
Instead of:
