Add example winborder setting

[eamonburns]

By default, there are no borders around floating windows (e.g. the ones created by vim.lsp.buf.hover()). Some people may want to change that.

It would be useful to have a line in init.lua explaining the winborder option, and setting it to some value (even just "none").

-- Default border style of floating windows.
-- See `:h 'winborder'`
vim.o.winborder = "none"