README: more links

README.md

@@ -2,15 +2,19 @@
 
 [![Build status](https://badge.buildkite.com/79bd0a8aa1e47a92e0254ca3afe5f439776e6d389cfbde9d8c.svg?branch=master)](https://buildkite.com/tweag-1/rules-nixpkgs)
 
-Use Nixpkgs to import external dependencies (like system packages)
-into Bazel hermetically. If the version of any dependency changes,
-Bazel will correctly rebuild targets, and only those targets that use
-the external dependencies that changed.
+Use [Nix][nix] and the [Nixpkgs][nixpkgs] package set to import
+external dependencies (like system packages) into [Bazel][bazel]
+hermetically. If the version of any dependency changes, Bazel will
+correctly rebuild targets, and only those targets that use the
+external dependencies that changed.
 
 Links:
 * [Nix + Bazel = fully reproducible, incremental builds][blog-bazel-nix]
 * [Nix + Bazel lightning talk][youtube-bazel-nix]
 
+[nix]: https://nixos.org/nix
+[nixpkgs]: https://github.com/NixOS/nixpkgs
+[bazel]: https://bazel.build
 [blog-bazel-nix]: https://www.tweag.io/posts/2018-03-15-bazel-nix.html
 [youtube-bazel-nix]: https://www.youtube.com/watch?v=hDdDUrty1Gw