Add online help functions for plugin window about main

[jwu]

Please check https://github.com/mbbill/undotree/blob/master/plugin/undotree.vim#L15, The way he wrote help is using the

"Custom key mappings: add this function to your vimrc.
"You can define whatever mapping as you like, this is a hook function which
"will be called after undotree window initialized.
"
"function g:undotree_CustomMap()
"    map <c-n> J
"    map <c-p> K
"endfunction

"=================================================
" Help text
let s:helpmore = ['"    ===== Marks ===== ',
            \'" >num< : current change',
            \'" {num} : change to redo',
            \'" [num] : the last change',
            \'"   s   : saved changes',
            \'"   S   : last saved change',
            \'"   ===== Hotkeys =====']
let s:helpless = ['" Press ? for help.']

" Keymap
let s:keymap = []
" action, key, help.
let s:keymap += [['Help','?','Toggle quick help']]
let s:keymap += [['Close','q','Close this panel']]
let s:keymap += [['FocusTarget','<tab>','Set Focus to editor']]
let s:keymap += [['ClearHistory','C','Clear undo history']]
let s:keymap += [['TimestampToggle','T','Toggle relative timestamp']]
let s:keymap += [['DiffToggle','D','Toggle diff panel']]
let s:keymap += [['GoNext','K','Revert to next state']]
let s:keymap += [['GoPrevious','J','Revert to previous state']]
let s:keymap += [['Redo','<c-r>','Redo']]
let s:keymap += [['Undo','u','Undo']]
let s:keymap += [['Enter','<2-LeftMouse>','Revert to current']]
let s:keymap += [['Enter','<cr>','Revert to current']]

then call

function! s:undotree.BindKey()
    for i in s:keymap
        silent exec 'nnoremap <silent> <script> <buffer> '.i[1].' :call <sid>undotreeAction("'.i[0].'")<cr>'
    endfor
    if exists('*g:undotree_CustomMap')
        call g:undotree_CustomMap()
    endif
endfunction