Add Koalas project in pandas Ecosystem

[HyukjinKwon]

Code Sample, a copy-pastable example if possible

N/A

Problem description

There is a project called Koalas which targets the distributed Pandas DataFrame on the top of Apache Spark.

It might be good to document this as one of the eco systems too.

Expected Output

Koalas description at https://pandas.pydata.org/pandas-docs/stable/ecosystem.html

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.7.3.final.0 python-bits: 64 OS: Darwin OS-release: 18.5.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

pandas: 0.24.2
pytest: 4.4.1
pip: 19.0.3
setuptools: 40.8.0
Cython: None
numpy: 1.16.2
scipy: None
pyarrow: 0.13.0
xarray: None
IPython: None
sphinx: 2.0.1
patsy: None
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None