document lsp_buffer_enabled in README.md (#627)

README.md

@@ -22,6 +22,20 @@ if executable('pyls')
         \ 'whitelist': ['python'],
         \ })
 endif
+
+function! s:on_lsp_buffer_enabled() abort
+    setlocal omnifunc=lsp#complete
+    setlocal signcolumn=yes
+    nmap <buffer> gd <plug>(lsp-definition)
+    nmap <buffer> <f2> <plug>(lsp-rename)
+    " refer to doc to add more commands
+endfunction
+
+augroup lsp_install
+    au!
+    " call s:on_lsp_buffer_enabled only for languages that has the server registered.
+    autocmd User lsp_buffer_enabled call s:on_lsp_buffer_enabled()
+augroup END
 ```
 
 Refer to [vim-lsp-settings](https://github.com/mattn/vim-lsp-settings) on how to easily setup language servers using vim-lsp automatically.