doc: explain why GitHub merge button is not used

Adds documentation and explicit reasons on why
the GitHub web interface button is not used. This was
explained in the referenced issue by @TheAlphaNerd.

Fixes: https://github.com/nodejs/node/issues/8893
PR-URL: https://github.com/nodejs/node/pull/9044


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>