DOC: Add pyjanitor to ecosystem.rst
[pyjanitor](https://github.com/ericmjl/pyjanitor) is a quite nice pandas data cleaning library and gives its users some useful method on data frames. It's pipe oriented, so it can be helpful in data cleaning/transformation work.

Adding pyjanitor to the ecosystem page gives it visibility to our users and makes it easier for the users to discover if there is functionailty in pyjanitor, that will help their use cases.