docs: add separate install guide

INSTALL.md

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+# Index
+
+* [Index](#index)
+* [Installing from Souce](#installing-from-source)
+* [AUR](#aur)
+* [Nix Flake](#nix-flake)
+
+> \[!CAUTION]
+> I encourage you to read the manual installation steps to understand what will get installed in your computer by this package.
+
+# Installing from Source
+
+Firstly, you'll need to build the package, this also includes man pages, default config, themes and other files you might need.
+
+To build it you only need to have some basic packages (should come pre-installed in almost all distros): `make`, `gcc` or another `gcc`ish compiler, and libpam headers. If it builds, it surely works anyways.
+
+```sh
+git clone https://github.com/javalsai/lidm.git
+cd lidm
+make # 👍
+```
+
+> \[!NOTE]
+> Theres pre-built binaries on the [releases tab](https://github.com/javalsai/lidm/releases) too.
+
+Then you can install the files onto your filesystem with:
+
+```sh
+make install
+```
+
+And additionally, to install service files (start-up behavior). <sup>[more docs](./assets/services/README.md)</sup>
+
+```sh
+# automatically detects your init system
+# and install service file (for tty7)
+make install-service
+
+# or if you don't like autodetection
+make install-service-systemd # systemd
+make install-service-dinit # dinit
+make install-service-runit # runit
+make install-service-openrc # openrc
+make install-service-s6 # s6
+```
+
+# AUR
+[AUR packages](https://aur.archlinux.org/packages?K=lidm&SeB=n) will automatically install most files. 
+
+> [!CAUTION]
+> [service files](./assets/pkg/aur#services) have to be manually installed by now.
+
+# Nix Flake
+You can install by doing
+```sh
+nix profile install github:javalsai/lidm
+```
+or try it out without installing by:
+```sh
+nix run github:javalsai/lidm
+```
+
+> [!CAUTION]
+> This doesn't include [service files](./assets/pkg/aur#services) neither