Fix/Updates for debian documentation

Add information for building doxygen documentation
on debian and fix for hardcoded path for dot.

doc/README

@@ -12,3 +12,10 @@ $doxygen Doxyfile
 
 The html file index.html should be in doc/html. 
 
+Debian
+=====================
+#To build documentation first install dependencies (doxygen, doxyqml and dot):
+sudo apt install doxygen graphviz doxyqml
+
+#Then build documentation in the doc/ directory
+doxygen ./Doxyfile

doc/dot

@@ -34,4 +34,13 @@ do
   fi
 done
 
-/usr/local/bin/dot ${ARGS}
+if [[ -x "/usr/local/bin/dot" ]]  
+then
+  /usr/local/bin/dot ${ARGS}
+elif [[ -x "/usr/bin/dot" ]]
+then 
+  /usr/bin/dot ${ARGS}
+else
+  echo "Program dot not found: See file doc/dot"
+  exit 1
+fi