docs(project-nvim): add note about built-in functionality (#989)

lua/astrocommunity/project/project-nvim/README.md

@@ -3,3 +3,5 @@
 The superior project management solution for neovim.
 
 **Repository:** <https://github.com/jay-babu/project.nvim>
+
+_Note_: The functionality of this plugin is part of AstroNvim as of [v4.0.0](https://github.com/AstroNvim/AstroNvim/releases/tag/v4.0.0). To enable automatic `:cd` or chdir to project root, or to configure other options, see the [project rooter documentation](https://docs.astronvim.com/recipes/rooter/). Enabling this plugin will disable the built-in project rooter.