Change comments/READE.md in order to make clear that users need to take care for mutexing in case of multiple writer threads

In case of multiple writer threads, the user has to take care to use mutexes to prevent concurrent access to cabad, not cabad itself.

• All comments/doc needs to be updated accordingly
• The mutex introduced by db4fe7a5 probably can be reverted … was never a good solution. Possible mutexing will be discussed in a different issue.