map -> nmap; document "set defaults" in README

Opening the peekaboo markbar with `map` keybindings breaks visual mode
selection and pending operators. Only open the peekaboo markbar from
normal mode, so that users can still use mark motions from visual mode,
operator pending mode, etc.

Document this in the README.

README.md

@@ -76,11 +76,11 @@ Quick Start
 At a minimum, you should set some of the following keymappings in your `.vimrc`.
 
 ```vim
-map <Leader>m <Plug>ToggleMarkbar
+nmap <Leader>m <Plug>ToggleMarkbar
 
 " the following are unneeded if ToggleMarkbar is mapped
-map <Leader>mo <Plug>OpenMarkbar
-map <Leader>mc <Plug>CloseMarkbar
+nmap <Leader>mo <Plug>OpenMarkbar
+nmap <Leader>mc <Plug>CloseMarkbar
 ```
 
 These examples use the [leader key,](https://stackoverflow.com/questions/1764263/what-is-the-leader-in-a-vimrc-file)
@@ -147,10 +147,6 @@ _jump to_ mark `a`.
 
 By default, you can **jump to the selected mark** by pressing the `Enter` key.
 
-As of the time of writing, **the peekaboo markbar is not compatible with visual
-mode,** e.g. you can't press `V` to start a linewise selection, and then press
-`'a` to select every line between that starting position and the mark `a`.
-
 Customization
 --------------------------------------------------------------------------------
 
@@ -182,9 +178,9 @@ let g:markbar_reset_mark_mapping    = 'r'
 let g:markbar_delete_mark_mapping   = '<Del>'
 
 " open/close markbar mappings
-map <Leader>m  <Plug>ToggleMarkbar
-map <Leader>mo <Plug>OpenMarkbar
-map <Leader>mc <Plug>CloseMarkbar
+nmap <Leader>m  <Plug>ToggleMarkbar
+nmap <Leader>mo <Plug>OpenMarkbar
+nmap <Leader>mc <Plug>CloseMarkbar
 ```
 
 ### Peekaboo Customization
@@ -205,6 +201,22 @@ let g:markbar_peekaboo_apostrophe_mapping = '<leader>a'
 let g:markbar_peekaboo_backtick_mapping = '<leader>b'
 ```
 
+Alternatively, if you want to set your own explicit mappings, you can set the
+following options:
+
+```vim
+" leave peekaboo markbar enabled, but don't set default mappings
+let g:markbar_set_default_peekaboo_mappings = v:false
+
+nmap <leader>a <Plug>OpenMarkbarPeekabooApostrophe
+nmap <leader>b <Plug>OpenMarkbarPeekabooBacktick
+```
+
+As of the time of writing, the peekaboo markbar isn't fully compatible with
+operator-pending mode, visual mode, or select mode. For instance, setting `vmap
+' <Plug>OpenMarkbarPeekabooApostrophe` will allow you to _open_ the
+peekaboo markbar from visual mode, but doing so will cancel your visual selection.
+
 If you want to disable the peekaboo markbar entirely, you can set
 `g:markbar_enable_peekaboo` to `v:false`.
 

plugin/vim-markbar.vim

@@ -20,9 +20,9 @@ if markbar#settings#EnablePeekabooMarkbar()
     noremap <silent> <Plug>OpenMarkbarPeekabooBacktick      :call g:peekaboo_controller.backtick()<cr>
 
     if markbar#settings#SetDefaultPeekabooMappings()
-        execute 'map <silent> '.markbar#settings#PeekabooApostropheMapping()
+        execute 'nmap <silent> '.markbar#settings#PeekabooApostropheMapping()
             \ .' <Plug>OpenMarkbarPeekabooApostrophe'
-        execute 'map <silent> '.markbar#settings#PeekabooBacktickMapping()
+        execute 'nmap <silent> '.markbar#settings#PeekabooBacktickMapping()
             \ .' <Plug>OpenMarkbarPeekabooBacktick'
     endif
 endif