Add placeholder std feature

Cargo.toml

@@ -8,3 +8,7 @@ description = "A better Box<dyn Error>"
 repository = "https://github.com/dtolnay/anyhow"
 documentation = "https://docs.rs/anyhow"
 readme = "README.md"
+
+[features]
+default = ["std"]
+std = []

src/lib.rs

@@ -4,6 +4,9 @@ mod as_dyn_error;
 mod context;
 mod error;
 
+#[cfg(not(feature = "std"))]
+compile_error!("no_std support is not implemented yet");
+
 pub use crate::as_dyn_error::AsDynError;
 pub use crate::context::Context;
 pub use crate::error::{Chain, Error};