Add Cargo (Rust) crosswalk page (#65)

- The crosswalk CSV is at https://github.com/codemeta/codemeta/blob/master/crosswalks/Cargo.csv
- The column name in https://github.com/codemeta/codemeta/blob/master/crosswalk.csv is "Rust Package Manager"
- The Cargo logo is from https://doc.rust-lang.org/cargo/

Author: bact

content/crosswalk/cargo.md

@@ -0,0 +1,13 @@
+---
+title: "Crosswalk for Rust Package Manager"
+image: "/img/cargo.png"
+date: "2020-03-05"
+---
+
+[Cargo](https://doc.rust-lang.org/cargo/) is the Rust package manager.
+
+The `Cargo.toml` file for each package is called its manifest. Cargo uses the metadata in the manifest to download Rust package’s dependencies, compile packages, make distributable packages, and upload them to crates.io, the Rust community’s package registry.
+
+The manifest format is described in [the Cargo Book](https://doc.rust-lang.org/cargo/reference/manifest.html).
+
+{{< crosswalk name="Rust Package Manager" >}}