Avoid merge conflicts in NEWS file

by using the `union` merge strategy.

See also http://krlmlr.github.io/using-gitattributes-to-avoid-merge-conflicts/.
Merge pull request #913 from krlmlr/gitattributes-NEWS

Avoid merge conflicts in NEWS file
Re-roxygenise
Tweaking checklist
add "none" to documentation of theme() for parameter legend.position

Fixes #829
update Rd file
update NEWS
First attempt at staticdocs auto-deploy
Merge pull request #918 from krlmlr/legend-position-doc-2

add "none" to documentation of theme() for parameter legend.position
Merge branch 'master' into feature/839-facet-margin-2