Add some notes.
diff --git a/src/os.rs b/src/os.rs
index c0010fb..8ee3606 100644
--- a/src/os.rs
+++ b/src/os.rs
@@ -28,7 +28,10 @@
///
/// This usually does not block. On some systems (e.g. FreeBSD, OpenBSD,
/// Max OS X, and modern Linux) this may block very early in the init
-/// process, if the CSPRNG has not seeded yet.
+/// process, if the CSPRNG has not been seeded yet.[1]
+///
+/// [1] See https://www.python.org/dev/peps/pep-0524/ for a more in-depth
+/// discussion.
pub struct OsRng(imp::OsRng);
impl OsRng {
