Add basic README document.

README.md

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+# lockfree-collector
+
+[![crates.io](https://img.shields.io/crates/v/lockfree-collector.svg)](https://crates.io/crates/lockfree-collector)
+[![docs.rs](https://docs.rs/lockfree-collector/badge.svg)](https://docs.rs/lockfree-collector)
+[![github.com](https://github.com/adamreichold/lockfree-collector/actions/workflows/test.yaml/badge.svg)](https://github.com/adamreichold/lockfree-collector/actions/workflows/test.yaml)
+
+A lock-free collector which uses blocking to amortize the cost of heap allocations and stealing to keep the collection phase efficient, especially if there no values need to be collected.
+
+## License
+
+Licensed under
+
+ * [Apache License, Version 2.0](LICENSE-APACHE) or
+ * [MIT license](LICENSE-MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.