Add comment to document on-the-fly ansible vault crypto

I once wrote a simple function to decrypt and encrypt ansible-vaults
on the fly. This change finally ands a short explanatory comment to it.

.config/nvim/lua/config/defaults.lua

@@ -175,6 +175,7 @@ vim.api.nvim_create_autocmd("FileType", {
 	end,
 })
 
+-- descrypt and encrypt ansible vault on the fly
 if vim.fn.executable('ansible-vault') then
 	local function file_exists(name)
 		local f = io.open(name, "r")