kamailio-kemi-framework: added file with details for KSR.tls.cget()

kamailio-kemi-framework/docs/modules/tls/tls.cget.md

@@ -0,0 +1,9 @@
+Return values corresponding the pseudo-variables exported by TLS module, related
+to TLS connection and certificates. The parameter has to be the name of the
+pseudo-variable (without `$`).
+
+Example:
+
+```
+local vPeerSubjectCn = KSR.tls.cget("tls_peer_subject_cn");
+```