Tell users where they can find the native API headers

The listed directories are the same as in PRESUBMIT.py, which
was previously the only way to look this stuff up...

BUG=webrtc:8445
NOTRY=true

Change-Id: I218c91e554f3163ae13fc75b9cbf76b42f70624e
Reviewed-on: https://webrtc-review.googlesource.com/16432
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20670}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 69751a8..5d9f459 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -93,6 +93,8 @@
   'voice_engine/include',
 )
 
+# NOTE: The set of directories in API_DIRS should be the same as those
+# listed in the table in native-api.md.
 API_DIRS = NATIVE_API_DIRS[:] + LEGACY_API_DIRS[:]
 
 # TARGET_RE matches a GN target, and extracts the target name and the contents.