Add a blurb about installing missing emoji on Ubuntu

Right next to the nerdfonts blurb as requested.

README.md

@@ -27,6 +27,7 @@ External Requirements:
 - Clipboard tool (xclip/xsel/win32yank or other depending on the platform)
 - A [Nerd Font](https://www.nerdfonts.com/): optional, provides various icons
   - if you have it set `vim.g.have_nerd_font` in `init.lua` to true
+- Emoji fonts (Ubuntu only, and only if you want emoji!) `sudo apt install fonts-noto-color-emoji`
 - Language Setup:
   - If you want to write Typescript, you need `npm`
   - If you want to write Golang, you will need `go`