doc: update readme

updated readme to include install/usage guide, requirements and theme list

README.md

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+# Signal Desktop Themer
+A script for injecting theme into [signal-desktop](https://github.com/signalapp/Signal-Desktop).
+## Requirements
+- python
+- python-pip
+- pipx
+## Installation/Usage
+**Note: Running this script requires sudo permission in linux. If you are unsure about this, the script is only a few lines and you can go thorough it really quickly.**
+1. Install applications in requirements section if you don't have them. (search online if you don't know how)
+2. Install this script with `pipx install signal-themer`
+3. Run `sudo signal-themer <path/to/theme.css>` in linux and `signal-themer <path\to\theme.css>` in windows.
+4. Launch Signal and enjoy!
+## Removal
+Currently there is no 'clean' way to remove the theme but it might be added in the future.
+For now, create an empty css file and use that as theme.
+## Theme List
+- [whatsapp](https://github.com/CapnSparrow/signal-desktop-themes)
+- [yuri and zero two](https://github.com/Foxunderground0/Signal-Themes)
+- [catppuccin](https://github.com/CalfMoon/signal-desktop)