docs: add how to accomplish packer's rtp (#259)

README.md

@@ -674,6 +674,12 @@ For a real-life example, you can check my personal dots:
 - `config` don't support string type, use `fun(LazyPlugin)` instead.
 - `module` is auto-loaded. No need to specify
 - `keys` spec is [different](#%EF%B8%8F-lazy-key-mappings)
+- `rtp` can be accomplished with:
+     ```lua
+         config = function(plugin)
+             vim.opt.rtp:append(plugin.dir .. "/rtpPath") 
+         end 
+     ```
 
 With packer `wants`, `requires` and `after` can be used to manage dependencies.
 With lazy, this isn't needed for most of the lua dependencies. They can be installed just like normal plugins