INSTALL: added link to the install tutorial on the web

INSTALL

@@ -18,6 +18,8 @@
     - https://www.kamailio.org/w/documentation/
     - https://www.kamailio.org/wiki/
 
+  Recommended install tutorial:
+    - http://kamailio.org/docs/tutorials/5.2.x/kamailio-install-guide-git/
 
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