Added beginnings of Coding-Style.md, fixed safe proposal based on latest discussion and fixed of adding a traversal tutorial with TRAVOpt and so on

tutorials/adding-a-traversal.md

@@ -241,6 +241,8 @@ There is also a template for your traversal `.c` file, here it is:
 * description:
 *   Longer traversal description goes here with examples.
 *
+* example: 
+*
 * @{
 **********************************************************/
 
@@ -333,6 +335,7 @@ TEMPmodule (node *arg_node, info *arg_info)
 {  
     DBUG_ENTER ();  
 
+  DBUG_ASSERT (NODE_TYPE (arg_node) == N_module, "called with non-module node");
 	MODULE_FUNS (arg_node) = TRAVopt (MODULE_FUNS (arg_node), arg_info);
   
     DBUG_RETURN (arg_node);  
@@ -410,6 +413,13 @@ You should define accessor macros for all the fields of your info struct.
 You can actually alter the AST by returning a modified node. 
 Think if you need to make a copy of the node before returning it.
 Special recursive copy functions are generated from `ast.xml` which can copy nodes correctly.
+
+## General Tips
+
+- Use `TRAVopt` instead of `TRAVdo` if the node you traverse is maybe `NULL`.
+- Flags should be a question. For example call them `IsSafe` instead of `Safe`.
+- The description of a traversal should be in the .c file because this is where the is.
+
 # Step 6: Register your phase in the compiler
 
 Once your traversal compiles, and you are happy, you can actually add it the compiler.