Missing documentation on logger

Currently there is no documentation in the logger and I struggle to use it. If I try to do:
```
auto tree = factory.createTreeFromText(xml_text);
BT::RosoutLogger rosout_logger(tree);
```
I get the error
```
error: no matching function for call to ‘BT::RosoutLogger::RosoutLogger(BT::Tree&)’
```