Add notes on re-building schema_capnp.rs to CONTRIBUTING.md
closes
@@ -101,3 +101,16 @@ latest stable.
```shell
rustup upgrade stable
```
+
+## Cap'n Proto schema
+
+The compiler uses a Cap'n Proto schema to serialize/deserialize module information.
+Occasionally, the schema needs to change. After modifying `compiler-core/schema.capnp`
+you need to to re-generate `compiler-core/generated/schema_capnp.rs`. To do that,
+[install Cap'n Proto](https://capnproto.org/install.html) and un-comment appropriate lines
+in `compiler-core/build.rs`. Then you should be able to re-generate that file with:
+
+```shell
+cd compiler-core
+cargo build
+```
