The docs are based on an internal google document available at
go/vangle-line-rasterization. The link won't work for non-Googlers.

Also cleans up the format of the markdown file somewhat.

Bug: angleproject:3344
Change-Id: Ic466c44099e25f12b9afc4565bbd0fe37097af9f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1561969Reviewed-by: Kenneth Russell <kbr@chromium.org>