repo: add .vscode to gitignore and add a few missing comments

.gitignore

@@ -8,3 +8,5 @@ Cargo.lock
 
 # These are backup files generated by rustfmt
 **/*.rs.bk
+
+.vscode

examples/scales_example.rs

@@ -7,6 +7,7 @@ use rust_music::{
     Instrument, Note, NoteName, Part, Phrase, Score, Tempo,
 };
 
+// This example requires the `composition` feature.
 fn main() -> Result<(), Box<dyn Error>> {
     // Create a simple C Minor Scale on octave 4 (this requires the `composition` feature)
     let s = Scale::new(

src/lib.rs

@@ -7,6 +7,9 @@ mod part;
 mod phrase;
 pub mod score;
 
+// The `composition` feature enables the composition module which contains various
+// utilities that simplify music composition, for example by streamlining the creation
+// of common musical structures using music theory.
 #[cfg(feature = "composition")]
 pub mod composition;