Add install.md with usage instructions for install.sh

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

system_setup/install.md

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+# OEMDRV Bootstrap — install.sh
+
+`install.sh` prepares a target machine for automated Fedora deployment. It shrinks an existing partition to carve out a dedicated **OEMDRV** partition, which Anaconda/Kickstart will detect automatically during installation.
+
+## What it does
+
+1. Lists all ext4 and btrfs partitions that have enough free space to be shrunk.
+2. Asks you to select one and shrinks it by **4 GiB**.
+3. Creates a new 4 GiB BTRFS partition labeled `OEMDRV` in the freed space.
+4. Mounts it to `/opt/sys_config` with `compress=zstd:6`.
+5. Clones this repository (depth 1) into `/opt/sys_config`.
+
+## Prerequisites
+
+- Run as **root** on the target machine (live system or installed OS).
+- The following tools must be present: `parted`, `e2fsck`, `resize2fs` or `btrfs-progs`, `mkfs.btrfs`, `git`, `curl`.
+- The partition you want to shrink must **not** be the root filesystem (`/`) and must have at least **4.5 GiB free**.
+- Network access to `gitea.dtext.online`.
+
+## Run directly from the repository
+
+```bash
+bash <(curl -fsSL https://gitea.dtext.online/obel1x/fedora-OEMDRV/raw/branch/main/system_setup/install.sh)
+```
+
+> The script requires root. Run the command above as root, or prefix with `sudo`.
+
+## After the script completes
+
+Configure your environment before running any installation:
+
+```sh
+cp /opt/sys_config/config/setup_system.conf.dist /opt/sys_config/config/setup_system.conf
+# Edit setup_system.conf — set TLDOMAIN, SERVERFQDN_IPA, SERVERFQDN_NC, and paths.
+```
+
+Optionally add local per-machine overrides in `config.d/`:
+
+```sh
+# Example: use the devel branch on this machine
+echo 'export UPGRADEBRANCH="devel"' > /opt/sys_config/config.d/system_defines.conf
+```
+
+Once configured, boot the Fedora installer from USB — Anaconda will detect the `OEMDRV` partition and run the Kickstart automatically.
+
+## Supported filesystems for shrinking
+
+| Filesystem | Method |
+|---|---|
+| ext4 | `e2fsck` + `resize2fs` (offline) |
+| btrfs | `btrfs filesystem resize` (temporary mount) |