Merge pull request rust-lang#348 from whitequark/patch-1

Document no_std support

README.md

@@ -31,6 +31,17 @@ For more information about how you can use futures with async I/O you can take a
 look at [https://tokio.rs](https://tokio.rs) which is an introduction to both
 the Tokio stack and also futures.
 
+### Feature `use_std`
+
+`futures-rs` works without the standard library, such as in bare metal environments.
+However, it has a significantly reduced API surface. To use `futures-rs` in
+a `#[no_std]` environment, use:
+
+```toml
+[dependencies]
+futures = { version = "0.1", default-features = false }
+```
+
 # License
 
 `futures-rs` is primarily distributed under the terms of both the MIT license and