Update README.md

passed a ``Featurizer`` object. ``deepchem`` provides a number of

different subclasses of ``Featurizer`` for convenience:

### Performances

|Dataset   	|Model               |Train score   |Valid score   |Time(loading)/s |Time(running)/s|

|-----------|--------------------|--------------|--------------|----------------|---------------| 

|tox21      |tensorflow(MT-DNN)  |0.987         |0.800         |35              |36 				|

|muv        |tensorflow(MT-DNN)  |0.979         |0.660         |414             |255            |

|pcba   		|tensorflow(MT-DNN)	|0.949        	|0.791         |1765         	  |7209           |                                         

## Contributing to DeepChem

We actively encourage community contributions to DeepChem. The first place to start getting involved is by running our examples locally. Afterwards, we encourage contributors to give a shot to improving our documentation. While we take effort to provide good docs, there's plenty of room for improvement. All docs are hosted on Github, either in this `README.md` file, or in the `docs/` directory.