Add more info subsection with links to forge and rustc api docs

Author: spastorino

src/about-this-guide.md

@@ -16,4 +16,11 @@ be found at the [GitHub repository]. If you find any mistakes in the
 guide, please file an issue about it, or even better, open a PR
 with a correction!
 
+## Other places to find information
+
+You are going to find [Rustc API docs site] useful and there's more
+supplementary documentation in [Rust Forge].
+
 [GitHub repository]: https://github.com/rust-lang/rustc-guide/
+[Rustc API docs site]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/
+[Rust Forge]: https://forge.rust-lang.org/