crates_io_tarball: Add README.md

Author: Turbo87

crates/crates_io_tarball/README.md

@@ -0,0 +1,13 @@
+# crates_io_tarball
+
+This package is used to extract metadata from a `.crate` file, which is the
+format used to distribute Rust libraries on https://crates.io.
+
+The main source of metadata is the `Cargo.toml` file, which must be included in
+the `.crate` file. 
+
+A secondary source of metadata is the `.cargo_vcs_info.json` file, which
+contains information about the version control system that was used at the
+time of publishing the crate. Note that this file is optional, and must not be
+relied upon for critical information since a malicious user could tamper with
+it before publishing the crate.

crates/crates_io_tarball/src/lib.rs

@@ -1,3 +1,5 @@
+#![doc = include_str!("../README.md")]
+
 #[cfg(test)]
 #[macro_use]
 extern crate claims;