Add more debugging tips to INSTALL file

INSTALL

@@ -41,6 +41,16 @@ Switch to your terminal, type bt ('backtrace') and hit Return.
 What you obtained now is a backtrace that should include
 function names and line numbers.
 
+If the problem is a warning and not a crash, try this:
+
+'G_DEBUG=all gdb _build_/default/midori/midori'
+
+If you are interested in HTTP communication, try this:
+
+'MIDORI_SOUP_DEBUG=2 _build_/default/midori/midori'
+
+Where '2' can be a level between 0 and 3.
+
 For further information a tutorial for gdb and
 reading up on how you can install debugging
 symbols for libraries used by Midori are recommended.