diff --git a/contrib/coccinelle/README b/contrib/coccinelle/README
index 9c2f887..f0e80bd 100644
--- a/contrib/coccinelle/README
+++ b/contrib/coccinelle/README
@@ -1,2 +1,43 @@
 This directory provides examples of Coccinelle (http://coccinelle.lip6.fr/)
 semantic patches that might be useful to developers.
+
+There are two types of semantic patches:
+
+ * Using the semantic transformation to check for bad patterns in the code;
+   The target 'make coccicheck' is designed to check for these patterns and
+   it is expected that any resulting patch indicates a regression.
+   The patches resulting from 'make coccicheck' are small and infrequent,
+   so once they are found, they can be sent to the mailing list as per usual.
+
+   Example for introducing new patterns:
+   67947c34ae (convert "hashcmp() != 0" to "!hasheq()", 2018-08-28)
+   b84c783882 (fsck: s/++i > 1/i++/, 2018-10-24)
+
+   Example of fixes using this approach:
+   248f66ed8e (run-command: use strbuf_addstr() for adding a string to
+               a strbuf, 2018-03-25)
+   f919ffebed (Use MOVE_ARRAY, 2018-01-22)
+
+   These types of semantic patches are usually part of testing, c.f.
+   0860a7641b (travis-ci: fail if Coccinelle static analysis found something
+               to transform, 2018-07-23)
+
+ * Using semantic transformations in large scale refactorings throughout
+   the code base.
+
+   When applying the semantic patch into a real patch, sending it to the
+   mailing list in the usual way, such a patch would be expected to have a
+   lot of textual and semantic conflicts as such large scale refactorings
+   change function signatures that are used widely in the code base.
+   A textual conflict would arise if surrounding code near any call of such
+   function changes. A semantic conflict arises when other patch series in
+   flight introduce calls to such functions.
+
+   So to aid these large scale refactorings, semantic patches can be used.
+   However we do not want to store them in the same place as the checks for
+   bad patterns, as then automated builds would fail.
+   That is why semantic patches 'contrib/coccinelle/*.pending.cocci'
+   are ignored for checks, and can be applied using 'make coccicheck-pending'.
+
+   This allows to expose plans of pending large scale refactorings without
+   impacting the bad pattern checks.