Instructions for updating Drill Website

[Volodymyr Vysotskyi <vo...@apache.org>]

Hi all,

I have updated the instruction for updating the Apache Drill Website, so
you shouldn't face issues I have seen when was attempting to make some
changes there.

The instruction is placed in *README.md* file in *gp-pages* branch:
https://github.com/apache/drill/blob/gh-pages/README.md

Also, it would be good if devs whose changes have doc-impacting, update
site documentation, especially when adding new plugins, functionality, etc.
But don't forget to specify that this functionality is available only
starting from a specific Drill version.

Kind regards,
Volodymyr Vysotskyi