[hit-test & anchors] Explainers for hit-test and anchors

*Hello everyone,Initial versions of explainers for hit-test
<https://github.com/immersive-web/hit-test/blob/master/explainer.md> and
anchors <https://github.com/immersive-web/anchors/blob/master/explainer.md>
have just been added to their respective repos. Please feel free to start
creating issues or PRs to get the discussion flowing on these topics. Also,
remember to watch these (and other) repos that you are interested in so
that you’ll receive updates on PRs and issues.Thank you.*