Add a README

Author: alexcrichton

README.md

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+# dlmalloc-rs
+
+A port of [dlmalloc] to Rust.
+
+[![Build Status](https://travis-ci.com/alexcrichton/dlmalloc-rs.svg?branch=master)](https://travis-ci.com/alexcrichton/dlmalloc-rs)
+
+[Documentation](https://docs.rs/dlmalloc)
+
+[dlmalloc]: http://g.oswego.edu/dl/html/malloc.html
+
+## Why dlmalloc?
+
+This crate is a port of [dlmalloc] to Rust, and doesn't rely on C. The primary
+purpose of this crate is to serve as the default allocator for Rust on the
+`wasm32-unknown-unknown` target. At the time this was written the wasm target
+didn't support C code, so it was required to have a Rust-only solution.
+
+This allocator is not the most performant by a longshot. It is primarily, I
+think, intended for being easy to port and easy to learn. I didn't dive too deep
+into the implementation when writing it, it's just a straight port of the C
+version.
+
+It's unlikely that Rust code needs to worry/interact with this allocator in
+general. Most of the time you'll be manually switching to a different allocator
+:)
+
+# License
+
+This project is licensed under either of
+
+ * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+   http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+   http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in this project by you, as defined in the Apache-2.0 license,
+shall be dual licensed as above, without any additional terms or conditions.