esp32: Add some notes about the different CMake files.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
diff --git a/ports/esp32/esp32_common.cmake b/ports/esp32/esp32_common.cmake
index 48f067d..9d51a03 100644
--- a/ports/esp32/esp32_common.cmake
+++ b/ports/esp32/esp32_common.cmake
@@ -1,3 +1,9 @@
+# This is the common ESP-IDF "main component" CMakeLists.txt contents for MicroPython.
+#
+# This file is included directly from a main_${IDF_TARGET}/CMakeLists.txt file
+# (or included from an out-of-tree main component CMakeLists.txt for out-of-tree
+# builds.)
+
# Set location of base MicroPython directory.
if(NOT MICROPY_DIR)
get_filename_component(MICROPY_DIR ${CMAKE_CURRENT_LIST_DIR}/../.. ABSOLUTE)