Add example to override = mappings in codefmt docs

[dbarnett]

Some filetypes have terrible default indenting logic (using the = mapping) and also have a formatter fast enough and robust enough to drop in as the formatting implementation for the = mapping. You can do this by adding something like the following to your vimrc:

function s:AddCodefmtEqualMapping() abort
  " Replace all the various ={motion} keys to codefmt
  nnoremap <buffer> = :set opfunc=codefmt#FormatMap<CR>g@
  nnoremap <buffer> == :FormatLines<CR>
  vnoremap <buffer> = :FormatLines<CR>
endfunction
augroup codefmt_equal
  autocmd FileType cpp,proto call s:AddCodefmtEqualMapping()
augroup END

We should add an example like this into the codefmt help and explain what to expect + caveats when you use it.