dev: added README for tmux themes

extra/tmux/README.md

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+# Tmux Themes
+
+Making your Tmux multiplexer colors match your Catppucino colors is easy! Simply copy the `.conf` file into your `.tmux.conf` (which should be located under `$HOME`).
+
+## Base config
+
+In this directory you'd also find a [base config for Tmux](<pending>) with useful comment to get you started if you don't already have one. 
+
+## Alacritty/Kitty Incompatibility
+
+If you are having issues with the colors not displaying accordingly, see the [README from the Catppucino themes for Alacritty](https://github.com/Pocco81/Catppuccino.nvim/tree/main/extra/alacritty)