Add vsnip#completefunc to document

Author: hokorobi

README.md

@@ -23,6 +23,9 @@ VSCode(LSP)'s snippet feature in vim/nvim.
     - [vimcomplete](https://github.com/girishji/vimcomplete)
     - [ddc.vim](https://github.com/Shougo/ddc.vim)
     - [vim-easycompletion](https://github.com/jayli/vim-easycomplete)
+- Support built-in completion
+  - A function can be registered for finding completions.
+    `set complete+=Fvsnip#completefunc`
 - Vim script interpolation
   - You can use Vim script interpolation as `${VIM:...Vim script expression...}`.
 - SnipMate-like syntax support

doc/vsnip.txt

@@ -117,7 +117,12 @@ FUNCTION                                                      *vsnip-function*
 
     Register your own custom variable resolver.
 
+  vsnip#completefunc()
 
+    Register for finding completions.
+>
+    set complete+=Fvsnip#completefunc
+<
 
 ==============================================================================
 MAPPING                                                        *vsnip-mapping*