Updating CHANGELOG for 1.8.2rc2

Updating CHANGELOG for 1.8.2rc1

Adding .readthedocs.yml

fix timeout issues with readthedocs

Merge branch 'v1-8-stable' of https://git-wip-us.apache.org/repos/asf…

…/incubator-airflow into v1-8-stable

1.8.0 Release

Also tag 1.8.0+apache.incubating

Adding .readthedocs.yml to build docs for 1.7.1.3

Adding .readthedocs.yml to build docs for 1.7.1.3

Adding .readthedocs.yml to build versioned docs