README is not correct about how to include the generated code

[tgrushka]

When I follow your instructions and create this file:

// src/protocol.rs

include!(concat!(env!("OUT_DIR"), "/protocol.rs"));

I get this error:

cargo build                                                main 
   Compiling testclient v0.1.0 ...
error: environment variable `OUT_DIR` not defined at compile time
 --> testclient/src/protocol.rs:1:18
  |
1 | include!(concat!(env!("OUT_DIR"), "/protocol.rs"));
  |                  ^^^^^^^^^^^^^^^
  |
  = help: Cargo sets build script variables at run time. Use `std::env::var("OUT_DIR")` instead
  = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `testclient` (bin "testclient") due to 1 previous error

But when I do that, I obviously get the error:

1 | include!(concat!(std::env::var("OUT_DIR"), "/protocol.rs"));
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: only literals (like `"foo"`, `-42` and `3.14`) can be passed to `concat!()`

How do I use this project?