docs: add install guide

docs/install.md

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+# Self-hosting Slash with Docker
+
+Slash is designed for self-hosting through Docker. No Docker expertise is required to launch your own instance. Just basic understanding of command line and networking.
+
+## Requirements
+
+The only requirement is a server with Docker installed.
+
+## Docker Run
+
+To deploy Slash using docker run, just one command is needed:
+
+```bash
+docker run -d --name slash --publish 5231:5231 --volume ~/.slash/:/var/opt/slash stevenlgtm/slash:latest
+```
+
+This will start Slash in the background and expose it on port `5231`. Data is stored in `~/.slash/`. You can customize the port and data directory.
+
+## Upgrade
+
+To upgrade Slash to latest version, stop and remove the old container first:
+
+```bash
+docker stop slash && docker rm slash
+```
+
+It's recommended but optional to backup database:
+
+```bash
+cp -r ~/.slash/slash_prod.db ~/.slash/slash_prod.db.bak
+```
+
+Then pull the latest image:
+
+```bash
+docker pull stevenlgtm/slash:latest
+```
+
+Finally, restart Slash by following the steps in [Docker Run](#docker-run).