Add example to override = mappings in codefmt docs

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:

``` VimL
function s:AddCodefmtEqualMapping() abort
" Replace all the various ={motion} keys to codefmt
nnoremap = :set opfunc=codefmt#FormatMapg@
nnoremap == :FormatLines
vnoremap = :FormatLines
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.

Start with the codefmt help documentation and its existing formatting guidance. Add the supplied VimL example for overriding the = mappings, then document what users should expect and the caveats of applying it to selected filetypes; confirm the example is readable and consistent with the surrounding help text.

Written by the indexing model from the issue text.