Add docs for integarting copilot.vim

doc/cmp.txt

@@ -645,6 +645,31 @@ You can setup buffer specific configuration like this.
   })
 <
 
+How to integrate with copilot.vim?~
+
+  The copilot.vim and nvim-cmp both have a `key-mapping fallback` mechanism.
+  Therefore, You should manage those plugins by yourself.
+
+  Fortunately, the copilot.vim has the feature that disables the fallback mechanism.
+>
+  let g:copilot_no_tab_map = v:true
+  imap <expr> <Plug>(vimrc:copilot-dummy-map) copilot#Accept("\<Tab>")
+<
+  You can manage copilot.vim's accept feature with nvim-cmp' key-mapping configuration. 
+>
+  cmp.setup {
+    mapping = {
+      ['<C-g>'] = cmp.mapping(function(fallback)
+        vim.api.nvim_feedkeys(vim.fn['copilot#Accept'](vim.api.nvim_replace_termcodes('<Tab>', true, true, true)), 'n', true)
+      end)
+    },
+    experimental = {
+      ghost_text = false -- this feature conflict to the copilot.vim's preview.
+    }
+  }
+<
+
+
 How to customize menu appearance?~
 
   You can see the nvim-cmp wiki (https://github.com/hrsh7th/nvim-cmp/wiki).