Document the new feature

Author: svenfoo

test-generator/src/lib.rs

@@ -61,6 +61,15 @@
 //!
 //! test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
 //! ```
+//! ## Annotating tests to be ignored
+//! Sometimes tests should not be run by default, for example if they take a very
+//! long time to run. Such tests would usually be annotated with #[ignore].
+//! The `test_resources` macro allows to do this by adding the `ignore` keyword
+//! after the resource:
+//! ```ignore
+//! #[test_resources("res/*/input.txt", ignore)]
+//! ```
+//!
 //! # Example usage `bench`:
 //!
 //! The `bench` functionality requires the nightly release of the Rust-compiler.