Revise readme to guide users customize key bindings

This is a humble pull request, to give users a quick customization information, otherwise for people who have key conflicts with `<leader>r`, `<C-c>g`, etc, will be able to quickly customize the shortcuts themselves.

Take me as an example, I used <leader>r to do selected string replacement, by having the following contents in my `vimrc`:

    function! CmdLine(str)
        exe "menu Foo.Bar :" . a:str
        emenu Foo.Bar
        unmenu Foo
    endfunction

    function! VisualSelection(direction) range
        let l:saved_reg = @"
        execute "normal! vgvy"

        let l:pattern = escape(@", '\\/.*$^~[]')
        let l:pattern = substitute(l:pattern, "\n$", "", "")

        if a:direction == 'b'
            execute "normal ?" . l:pattern . "^M"
        elseif a:direction == 'gv'
            call CmdLine("vimrep " . '/'. l:pattern . '/' . ' **/*.')
        elseif a:direction == 'replace'
            call CmdLine("%s" . '/'. l:pattern . '/')
        elseif a:direction == 'f'
            execute "normal /" . l:pattern . "^M"
        endif

        let @/ = l:pattern
        let @" = l:saved_reg
    endfunction

    " Visual mode pressing * or # searches for the current selection
    " Super useful! From an idea by Michael Naumann
    vnoremap <silent> * :call VisualSelection('f')<CR>
    vnoremap <silent> # :call VisualSelection('b')<CR>
    " When I press <leader>r I can search and replace the selected text
    vnoremap <silent> <leader>r :call VisualSelection('replace')<CR>



I read the source code and found default key shortcuts definition is located in:  `python-mode/plugin/pymode.vim`, then I managed to override the default key bindings, this is indeed less convenient for users like me, thus I enhanced the readme.rst to include Customization information, please consider approve my pull request, thank you and appreciate your great work for this awesome plugin:)