tonic-build: (docs) adds NixOS related hints (#444)

tonic-build/src/lib.rs

@@ -40,7 +40,22 @@
 //!         )?;
 //!    Ok(())
 //! }
+//!```
+//!
+//! ## NixOS related hints
+//!
+//! On NixOS, it is better to specify the location of `PROTOC` and `PROTOC_INCLUDE` explicitly.
+//!
+//! ```bash
+//! $ export PROTOBUF_LOCATION=$(nix-env -q protobuf --out-path --no-name)
+//! $ export PROTOC=$PROTOBUF_LOCATION/bin/protoc
+//! $ export PROTOC_INCLUDE=$PROTOBUF_LOCATION/include
+//! $ cargo build
 //! ```
+//!
+//! The reason being that if `prost_build::compile_protos` fails to generate the resultant package,
+//! the failure is not obvious until the `include!(concat!(env!("OUT_DIR"), "/resultant.rs"));`
+//! fails with `No such file or directory` error.
 
 #![recursion_limit = "256"]
 #![warn(