[Advanced Notice] Important change on default branch of immersive-web repos (15-16/Jan/2021)

   hi all (in both IWWG/IWCG),

   Last year, we had several conversations on changing default branch name of immersive-web
github repositories [1], from master to main. And we will take this change over all IW
repositories this weekend, from this Fri (2021/Jan/15).
*1 https://github.com/immersive-web/administrivia/issues/107
# Assuming work will end in the day, but let me mark weekend (Jan/16-17) as spare.

   Following actions will be taken:

- [all] new main branch will start from current HEAD of master branch
- [repos for website, documents etc.] gh-pages will be served from main but not master
     (note; specs are using gh-pages, with automatic update via CI tool, and no change)
- [all] PRs will be added to upgrade CI to meet with this change
     (note; repos with TravisCI could get PR to use GitHub Actions)
- existing PRs will be rebased to HEAD of main (= current HEAD of master), or warning will be left if collided


   These works will take some amount of time (not short as an hour,, could be), so please
check each status per repository, when you update or add PR(s) from this Fri to weekend.
Also please note:

- before starting your work on new PR(s), check your local conditions, not to add against master after this
- before updating PR(s), update your local copy


   If you have any question/comment, please include chairs' list (immersive-web-chairs@w3.org)
especially on using direct email.

   best,