doc: Mention tag wildcard support

ref: https://github.com/wbthomason/packer.nvim/pull/809

README.md

@@ -44,7 +44,9 @@ use {
     config = function()
         require('neogen').setup {}
     end,
-    requires = "nvim-treesitter/nvim-treesitter"
+    requires = "nvim-treesitter/nvim-treesitter",
+    -- Uncomment next line if you want to follow only stable versions
+    -- tag = "*" 
 }
 ```