[img-conversion] Add explainer document (#7)

Explainer document summarising why the proposal is necessary.
You can view, comment on, or merge this pull request online at:

  https://github.com/WICG/img-conversion/pull/7

-- Commit Summary --

  * Add explainer document

-- File Changes --

    A explainer.md (43)

-- Patch Links --

https://github.com/WICG/img-conversion/pull/7.patch
https://github.com/WICG/img-conversion/pull/7.diff

---
Reply to this email directly or view it on GitHub:
https://github.com/WICG/img-conversion/pull/7