Add crates.io keywords and categories (#33)

Add `keywords` and `categories` to the package metadata to improve
discoverability on crates.io, as requested in #33. "json" in particular
is a commonly searched and filtered keyword.

Author: Diggsey

Cargo.toml

@@ -7,6 +7,8 @@ readme = "README.md"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/Diggsey/ijson"
 description = "A more memory efficient replacement for serde_json::Value"
+keywords = ["json", "serde", "serde_json", "memory-efficient", "value"]
+categories = ["data-structures", "encoding"]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 [features]