Add FAQ about testing kickstart.nvim

Something I ran into when testing kickstart.nvim with my current config
was a message on startup: "No specs found for module custom.plugins".
This is also the subject of several issues.

Hopefully this FAQ explains the limitation of testing kickstart.nvim with
an existing config you want to preserve in place.

README.md

@@ -132,6 +132,12 @@ Each PR, especially those which increase the line count, should have a descripti
 * Are there any cool videos about this plugin?
   * Current iteration of kickstart (coming soon)
   * Here is one about the previous iteration of kickstart: [video introduction to Kickstart.nvim](https://youtu.be/stqUbv-5u2s). Note the install via init.lua no longer works as specified. Please follow the install instructions in this file instead as they're up to date.
+* Can I test out this configuration and keep my current config? Yes, kind of.
+  * Clone or otherwise get this repo on your system e.g. `~/code/sandbox/kickstart.nvim`
+  * Start neovim using that file: `nvim -u ~/code/sandbox/kickstart.nvim/init.lua`
+  * NOTE: If your current config uses Lazy you may want to `Clean (X)` existing plugins
+  * NOTE: To add plugins you are limited to modifying `init.lua` as the files in `custom/plugins/` won't be available to neovim because the `-u` option doesn't change neovim's CONFIG DIRECTORY.
+  * NOTE: If you see a message this message at startup: "No specs found for module custom.plugins" you are attempting to customize neovim outside it's CONFIG DIRECTORY.
 
 ### Windows Installation