Move contribution guide to the official documentation site

[HyukjinKwon]

This PR moves the contribution guide at ./CONTRIBUTING.md to our official documentation.
pandas seems doing the similar stuff as well (https://pandas.pydata.org/pandas-docs/stable/development/contributing.html)

[codecov-io]

Codecov Report

Merging #841 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #841   +/-   ##
=======================================
  Coverage   94.17%   94.17%           
=======================================
  Files          32       32           
  Lines        6007     6007           
=======================================
  Hits         5657     5657           
  Misses        350      350

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[ueshin]

Otherwise, LGTM.

[HyukjinKwon]

Merged!

[vgangaprasad]

This PR moves the contribution guide at ./CONTRIBUTING.md to our official documentation.
pandas seems doing the similar stuff as well (https://pandas.pydata.org/pandas-docs/stable/development/contributing.html)

Hi HyukjinKwon, contributing link is not working, please check (https://koalas.readthedocs.io/en/latest/development/contributing.html)

[vgangaprasad]

This PR moves the contribution guide at ./CONTRIBUTING.md to our official documentation.
pandas seems doing the similar stuff as well (https://pandas.pydata.org/pandas-docs/stable/development/contributing.html)

Hi HyukjinKwon, contributing link is not working, please check (https://koalas.readthedocs.io/en/latest/development/contributing.html)

Ignore my message, it works, for some reason it didnt work first time. Sorry :(

[HyukjinKwon]

It took a while to build and render. Thanks for double checking this :D.