Update README.md

README.md: Fixed description of lib
Moved the conversion library from nrmllib to oq-commons
Added a note about the dependencies
Renamed commons to common
Fixed the Table.insert method
Changes on the fragility model converters to avoid denormalization
The exposure model is now fully supported
Fixed the exposure population test
Fixed few bugs, including a serious one for non-unique locations
Removed the "hidden" recordtype feature, which is better done on the Qt side
Changed the ordering of the fields in the Asset record
Added a convenient Converter.from_nrml classmethod
Improved validation and some cleanup
Add some validations
Fixed pep8
Add get_field_index
Removed properties that were used for plotting the record
Fix typo
Add (failing) test to check insert_update duplicating keys
Add record-level constraints and tests