Added release notes

docs/release_notes/v4.0.0.md

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+# SURFnet SURFconext EngineBlock v4.0.0 Release Notes #
+
+SimpleSAMLphp integration release.
+
+In this release we switched to using SimpleSAMLphp/saml2 library as an API for all SAML related funcationality as well
+as signing / verification of signatures.
+
+Notable other issues resolved with this release:
+* Security vulnerability in discovery screen.
+* Moved Mock IdP and SP to OpenConext-engine-test-stand
+* BACKLOG-1346: Made guest qualifier URN prefix configurable.
+* BACKLOG-1349: Many "session_destroy(): Trying to destroy uninitialized session" erros logged by EB
+* BACKLOG-1374: Feedback page not shown when "No a member of VO" error shown
+* BACKLOG-1371: EB stores logins without idp entityid
+* BACKLOG-1375: EB fails on setting original value of metadata attribute during normalization