Add README for Alacritty themes

extra/alacritty/README.md

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+# Alacritty Themes
+
+Making your Alacritty terminal emulator colors match your Catppucino colors is
+easy! Simply copy the values from the respective `.yml` file in this directory
+and paste it over the `colors` configuration in your `alacritty.yml` file.
+
+## `tmux` Users
+
+If you use `tmux` inside of Alacritty you may notice that your terminal colors
+don't match your Catppucino colors. The solution is two-fold. First, make sure
+you have the following set in `alacritty.yml`:
+
+```yml
+env:
+  TERM: xterm-256color
+```
+
+Second, make sure you have the following in your `tmux.conf`:
+
+```
+set -g default-terminal "xterm-256color"
+set-option -ga terminal-overrides ",xterm-256color:Tc"
+```
+
+Now you should be all set!
+
+See https://github.com/Pocco81/Catppuccino.nvim/pull/9 for more info.