doc: added a readme file to explain the role of doc folder

doc/README.md

@@ -0,0 +1,30 @@
+Kamailio - Doc Folder
+=====================
+
+The doc/ folder contains various resource used for generating documentation,
+as well as several tutotirals and info files from old times.
+
+Most of documentation for Kamailio can be found on the website:
+
+  * main index for documentation:
+    * https://www.kamailio.org/w/documentation/
+  * wiki site with cookbooks and tutorials:
+    * https://www.kamailio.org/wiki/
+  * documentation for the modules in latest stable version:
+    * https://www.kamailio.org/docs/modules/stable/
+
+Support Channels
+----------------
+
+Several mailing lists and an IRC channel is available for community interaction,
+see more about at:
+
+  * https://www.kamailio.org/w/support/
+
+Contact
+-------
+
+If you have further questions, you can contact the developers via sr-dev
+mailing list:
+
+  * sr-dev@lists.sip-router.org