add rust-lang/rust license documentation

Author: chrissimpkins

src/SUMMARY.md

@@ -32,6 +32,7 @@
     - [ICE-breaker teams](ice-breaker/about.md)
         - ["Cleanup Crew" ICE-breakers](ice-breaker/cleanup-crew.md)
         - [LLVM ICE-breakers](ice-breaker/llvm.md)
+    - [Licenses](./licenses.md)
 - [Part 2: How rustc works](./part-2-intro.md)
     - [High-level overview of the compiler source](./high-level-overview.md)
     - [The Rustc Driver and Interface](./rustc-driver.md)

src/licenses.md

@@ -0,0 +1,5 @@
+# Rustc Licenses
+
+The `rustc` compiler source is dual licensed under the [Apache License v2.0](https://github.com/rust-lang/rust/blob/master/LICENSE-APACHE) and the [MIT License](https://github.com/rust-lang/rust/blob/master/LICENSE-MIT) unless otherwise specified.
+
+Detailed licensing information is available in the [COPYRIGHT document](https://github.com/rust-lang/rust/blob/master/COPYRIGHT) of the `rust-lang/rust` repository.