DOC: ecosystem.rst: Vaex
Is it appropriate to add vaex to ecosystem.rst? What should the heading link and description be?

- Src: https://github.com/maartenbreddels/vaex
- Docs: https://docs.vaex.io/

>  Vaex is a python library for Out-of-Core DataFrames (similar to Pandas), to visualize and explore big tabular datasets. It can calculate statistics such as mean, sum, count, standard deviation etc, on an N-dimensional grid up to a billion (109) objects/rows per second. Visualization is done using histograms, density plots and 3d volume rendering, allowing interactive exploration of big data. Vaex uses memory mapping, zero memory copy policy and lazy computations for best performance (no memory wasted).

- https://github.com/maartenbreddels/vaex/blob/master/docs/source/getting_data_in_vaex.rst
  - vaex.from_pandas
  - vaex.to_pandas_df

https://github.com/pandas-dev/pandas/blob/master/doc/source/ecosystem.rst