Add more docs

crates/project_model/src/project_json.rs

@@ -1,4 +1,9 @@
-//! FIXME: write short doc here
+//! `rust-project.json` file format.
+//!
+//! This format is spiritually a serialization of [`base_db::CrateGraph`]. The
+//! idea here is that people who do not use Cargo, can instead teach their build
+//! system to generate `rust-project.json` which can be ingested by
+//! rust-analyzer.
 
 use std::path::PathBuf;