[#148] Add Geo category to Cargo.toml (#217)

- [x] I agree to follow the project's [code of
conduct](https://github.com/georust/.github/blob/main/CODE_OF_CONDUCT.md).
- [x] I added an entry to the project's change log file if knowledge of
this change could be valuable to users.
  - Usually called `CHANGES.md` or `CHANGELOG.md`
  - Prefix changelog entries for breaking changes with "BREAKING: "
---

---------

Co-authored-by: Laurențiu Nicola <lnicola@users.noreply.github.com>

Author: nk9

Cargo.toml

@@ -8,6 +8,7 @@ authors = [
 repository = "https://github.com/georust/proj"
 documentation = "https://docs.rs/proj/"
 keywords = ["proj", "projection", "osgeo", "geo"]
+categories = ["science::geo", "api-bindings"]
 license = "MIT OR Apache-2.0"
 readme = "README.md"
 edition = "2018"