Added readme

README.md

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+# furumi
+
+Terminal client (TUI) for the furumusic server. Cross-platform: Linux,
+macOS, Windows.
+
+## Building
+
+Rust 1.88+ (edition 2024).
+
+```bash
+cargo build --release      # binary: target/release/furumi
+```
+
+### Linux
+
+Sound output needs the system ALSA library — the one and only system
+build dependency (PipeWire/PulseAudio are reached through the ALSA
+compatibility layer at runtime):
+
+```bash
+# Debian/Ubuntu
+sudo apt install libasound2-dev pkg-config
+# Fedora
+sudo dnf install alsa-lib-devel pkgconf-pkg-config
+# Arch
+sudo pacman -S alsa-lib pkgconf
+```
+
+Everything else is pure Rust: TLS is rustls, MPRIS media keys go through
+zbus (no libdbus), images and audio decoding are Rust crates.
+
+### macOS / Windows
+
+No system packages required. (Windows: media keys are not wired up yet;
+playback works.)
+
+## Configuration
+
+- `~/.config/furumi/keymap.toml` — keybinding overrides, see
+  `src/config/default_keymap.toml` for the format and defaults.
+- `~/.config/furumi/credentials.json` — created on login (0600).
+- Logs: in-app on the Logs tab (`5`), and in the cache dir
+  (`furumi-cli.log`), filtered by `RUST_LOG`.