added tutorials for adding a traversal tutorial and a tutorial about project structure and ast.xml. Also added tutorials from other people I found in the source code and a concepts directory with phm and ref counting also added a faq file

tutorials/faq.md

+List of good things to know that are to short for their own file.
+
+- The `src/sac2clib/tree` directory has useful things.
+	- In `rename.c` there is a traversal which you can use to rename a name in the entire ast.
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