Show tab-based mapping in the README

[Julian]

Hi! (Thanks for making this!)

Other snippets plugins often show a quick snippet (ugh no pun intended) for how to "smart-map" <Tab> to either expand, jump to next slot, or insert a tab key (or ideally trigger completion).

Usually involving e.g. coc#expandableOrJumpable() for coc-snippets, or similar for vsnip, etc.

I'm sure there's likely a way to do so with snippets.nvim -- may be nice to have it in the README.

(Happy to send a PR if/when I figure out how to do so by jumping into the source).

[Julian]

OK, I found snippets.lookup_snippet_at_cursor which meant I ended up with:

-- Expand a snippet if it exactly matches, otherwise complete.
function tab()
  local _, expanded = snippets.lookup_snippet_at_cursor()
  if expanded ~= nil then
    snippets.expand_at_cursor()
    return
  end
  completion.smart_tab()
end

which I'm sure is still slightly inefficient because it expands the snippet twice but does seem to do what I want personally.