Add README for CLI tool

README.md

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+# Song Idea Generator
+
+Consisting of a Rust library for generating song ideas `libsig`,
+a command line tool for calling `libsig`,
+and a web interface with a JSON API for getting structured results from `libsig`.
+
+## Usage
+
+The primary type of `libsig` is the `SongIdea`. You can generate human-readable representations with `Display` (so, `println!("{}", my_song_idea)`).
+
+For ease of use there is a command line tool called `sigcli`:
+
+```
+$ cargo run --quiet --example cli -- 5
+skittery kick, dampened hat, and calm tom at 254 bpm, with no swing, through bitcrusher, reverb, phaser, distortion, and wah
+skittery bass, smooth pad, massive snare, smooth tom, and digital cymbal at 33 bpm, with heavy swing, clean
+splashy lead and harsh bass at 39 bpm, with no swing, through compressor
+buzzy pad, glassy vocals, plonky kick, plucky snare, and dry tom at 63 bpm, with heavy swing, through ring modulator
+surfy lead, blownout pad, light vocals, acidic snare, sonorous hat, and digital tom at 33 bpm, with no swing, through lofi
+```
+
+It also takes a parameter `--ambient` to avoid suggesting percussion parts.