Add cross-reference

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/docs/3.0-migration-guide.md b/docs/3.0-migration-guide.md
index 182a3bc..4d3ae57 100644
--- a/docs/3.0-migration-guide.md
+++ b/docs/3.0-migration-guide.md
@@ -597,7 +597,9 @@
 If you are working with the pre-V3 certificates you need to switch to the
 current ones.
 
+### Strengthen default algorithm selection for X.509
 
+This is described in the section (Strengthen default algorithm selection for X.509 and TLS)[strengthen-default-algorithm-selection-for-x.509-and-tls].
 
 ## SSL