Add keywords to Cargo package manifest

This was recommended by `lib.rs`'s maintainer dashboard, and it's
actually a good idea for making the crate more discoverable, I think.

Cargo.toml

@@ -2,6 +2,7 @@
 authors = ["Joshua Holmer <[email protected]>"]
 categories = ["command-line-utilities", "compression"]
 description = "A lossless PNG compression optimizer"
+keywords = ["png", "image-compression", "optimization", "multi-threading", "lossless"]
 documentation = "https://docs.rs/oxipng"
 edition = "2021"
 exclude = [