docs: add installation instructions for .deb, binary, and Nix

Document the three installation methods:
- Debian/Ubuntu .deb package with shell completions
- Statically-linked binary for any Linux system
- Nix flake and Home Manager module

README.md

@@ -4,6 +4,39 @@
 
 Strea.md is a personal knowledge management and time-tracking CLI tool. It organizes time-ordered markdown files using `@tag` annotations, letting you manage tasks, track time, and query your notes from the terminal.
 
+## Installation
+
+### Debian/Ubuntu (.deb package)
+
+Download and install the latest release:
+
+```bash
+wget https://git.konstantinfickel.de/kfickel/streamd/releases/download/vX.Y.Z/streamd_X.Y.Z_amd64.deb
+sudo dpkg -i streamd_X.Y.Z_amd64.deb
+```
+
+This includes shell completions for bash, zsh, and fish.
+
+### Static Binary
+
+Download the statically-linked binary:
+
+```bash
+wget https://git.konstantinfickel.de/kfickel/streamd/releases/download/vX.Y.Z/streamd-X.Y.Z-linux-x86_64
+chmod +x streamd-X.Y.Z-linux-x86_64
+sudo mv streamd-X.Y.Z-linux-x86_64 /usr/local/bin/streamd
+```
+
+### Nix
+
+Using the flake directly:
+
+```bash
+nix run git+https://git.konstantinfickel.de/kfickel/streamd
+```
+
+Or add to your NixOS/Home Manager configuration using the provided `homeManagerModules.default`.
+
 ## Core Concepts
 
 - **Shards** — Sections of markdown files, organized hierarchically by headings. Each shard can contain markers, tags, and nested child shards.