Clarify changelog not needed for compiler warnings

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/ChangeLog.d/00README.md b/ChangeLog.d/00README.md
index d2ea73d..2fbc989 100644
--- a/ChangeLog.d/00README.md
+++ b/ChangeLog.d/00README.md
@@ -21,6 +21,9 @@
 * Performance improvements, unless they are particularly significant.
 * Changes to parts of the code base that users don't interact with directly,
   such as test code and test data.
+* Fixes for compiler warnings. Releases typically contain a number of fixes
+  of this kind, so we will only mention them in the Changelog if they are
+  particularly significant.
 
 Until Mbed TLS 2.24.0, we required changelog entries in more cases.
 Looking at older changelog entries is good practice for how to write a