message: Add crates.io categories
  I noticed this crate didn't have any categories on crates.io. I figured at least `multimedia::images` and `multimedia::encoding` would be relevant.
diff: @@ -12,11 +12,13 @@ readme = "README.md"
  documentation = "https://docs.rs/image"
  repository = "https://github.com/PistonDevelopers/image.git"
  homepage = "https://github.com/PistonDevelopers/image"
  +categories = ["multimedia::images", "multimedia::encoding"]
  exclude = [
  "src/png/testdata/*",
  "examples/*",
  "tests/*",
  ]
  +
  [lib]
  name = "image"
  path = "./src/lib.rs"
  