Explain linking for upstream crates (based on #12575)

Author: aidanhs

src/doc/trpl/advanced-linking.md

@@ -5,6 +5,23 @@ but supporting the range of linking possibilities made available by other
 languages is important for Rust to achieve seamless interaction with native
 libraries.
 
+# Native libraries required by upstream crates
+
+Sometimes an upstream Rust crate won't include an appropriate `#[link(...)]`
+attribute to automatically link against a required natve library on your system.
+This can happen if libraries are named differently across platforms (so only
+you as the crate consumer know the actual name). To link against the appropriate
+library for your system, you can use the `#[link(...)]` attribute on an empty
+extern block:
+
+``` no_run
+extern crate foo;
+
+#[link(name = "foonative3"]
+extern {}
+# fn main() {}
+```
+
 # Link args
 
 There is one other way to tell rustc how to customize linking, and that is via