Adds Migration guide
Commit adds a migraiton guide entry that was
missing.
Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/docs/3.0-migration-guide.d/remove-truncated-HMAC-extension.md b/docs/3.0-migration-guide.d/remove-truncated-HMAC-extension.md
new file mode 100644
index 0000000..12f7c23
--- /dev/null
+++ b/docs/3.0-migration-guide.d/remove-truncated-HMAC-extension.md
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+Remove the truncated HMAC extension
+-----------------------------------
+
+This affects all users who use the truncated HMAC extension for cryptographic
+operations.
+
+The config option `MBEDTLS_SSL_TRUNCATED_HMAC` has been removed. Users concerned
+about overhead are better served by using any of the CCM-8 ciphersuites rather
+than a CBC ciphersuite with truncated HMAC, and so going forward this must be
+the approach taken.