Add fields to `memzero`'s Cargo.toml (#10362)

* Add fields to Cargo.toml before publishing to crates.io

* Change license from MIT to GPL-3.0

* Add docs page

util/memzero/Cargo.toml

@@ -1,4 +1,10 @@
 [package]
 name = "memzero"
 version = "0.1.0"
+description = "A wrapper for zero-ing out memory when dropped"
+license = "GPL-3.0"
+homepage = "https://parity.io"
+repository = "https://github.com/paritytech/parity-ethereum"
+docs = "https://docs.rs/crate/memzero"
 authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"