BufferQueue::push_back doc: pos argument is bytes, not chars

At least, Buffer.pos is documented as bytes, which is consistent with how it’s used.
Don’t count removed input as "available" in BufferQueue::push_back
Tweak README
Add a warning on OwnedDom
Merge remote-tracking branch 'origin/pr/36' into dev
Merge remote-tracking branch 'origin/pr/37' into dev
Upgrade to Rust 5d653c1
Add a regression test for #37