--- a/README	Wed Mar 11 15:55:36 2020 +0700
+++ b/README	Wed Mar 11 16:11:04 2020 +0700
@@ -139,6 +139,16 @@
     $ cd tests
     $ python $HGSRC/tests/run-tests.py
 
+When certain blocks of code need to cope with API changes in core Mercurial,
+they should have comments in the ``hg <= x.y (commit hash)`` format. For
+example, if a function needs another code path because of changes introduced in
+02802fa87b74 that was first included in Mercurial 5.3, then the comment should
+be::
+
+    # hg <= 5.2 (02802fa87b74)
+
+See also tests/test-check-compat-strings.t.
+
 Branch policy
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