Provide a section in the documentation that warns mptt users about the concurrency issues and about the fact that it's their responsibility for ensuring mutual exclusivity.

* this fixes issue #236
* also add a new link to an mptt tutorial (I find this more usefull than the existing ones as it also points out that CRUD operations need to be guarded by locking statements)
Merge pull request #242 from kux/issue_236

Documentation fix for issue #236