GRUB theme readme added

README.md

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+# Xenlism Grub Theme for NixOS
+
+## Installation
+
+- Add this flake input to your **flake.nix** inputs
+```nix
+inputs = {
+    grub-theme = {
+      url = "git+https://gitea.homework.zip/shinya/Xenlism-Grub-Theme";
+    };
+
+    # More inputs ...
+};
+```
+
+> [!IMPORTANT]
+> If you switched your theme from systemd-boot to GRUB
+> You have to change your boot entry in UEFI or BIOS to GRUB
+> Because NixOS will otherwise still boot into systemd-boot
+
+- Then set GRUB theme to this package in your **configuration.nix** file
+```nix
+boot.loader.grub.theme = inputs.grub-theme.packages.x86_64-linux.xenlism-grub-theme-nixos-1080p;
+    };
+};
+```
+
+> [!NOTE]
+> Just a side note, in case you are using stylix or something that sets GRUB theme for you
+> You will maybe have to set this option with *lib.mkForce* to make it override other value
+
+- You can pick from list of distros and resolutions that are present in the [original repo](https://github.com/xenlism/Grub-themes/)
+
+- To view all possible options run `nix flake show git+https://gitea.homework.zip/shinya/Xenlism-Grub-Theme --no-write-lock-file` this will display all possible packages to install
+
+- To change theme you just have to change the package name in **configuration.nix** option like this:
+```nix
+boot.loader.grub.theme = inputs.grub-theme.packages.x86_64-linux.xenlism-grub-theme-[distribution]-[resolution];
+```