Rename zcash_wallet to zcash_client_backend, set to 2018 edition

zcash_client_backend/README.md

@@ -0,0 +1,20 @@
+# zcash_client_backend
+
+This library contains Rust structs and traits for creating shielded Zcash light clients.
+
+## License
+
+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 the work by you, as defined in the Apache-2.0
+license, shall be dual licensed as above, without any additional terms or
+conditions.
+