add link to dart.dev FFI docs (#40)

Author: johnpryan

ffi/README.md

@@ -4,6 +4,9 @@ A series of simple examples demonstrating how to call C libraries from Dart.
 
 This code is designed to work with *Dart version 2.6.0* and above.
 
+To learn more about FFI, start with the [C interop using
+dart:ffi](https://dart.dev/guides/libraries/c-interop) guide on dart.dev.
+
 ## Building native libraries
 
 Each sample uses [CMake][cmake] to generate a Makefile. To build the native
@@ -36,4 +39,4 @@ The Dart binary can only load shared libraries that are *signed*. For more
 information, see [dart-lang/sdk/issues/38314][signing-issue] for details.
 
 [cmake]: https://cmake.org/
-[signing-issue]: https://github.com/dart-lang/sdk/issues/38314
+[signing-issue]: https://github.com/dart-lang/sdk/issues/38314

ffi/system-command/README.md

@@ -11,3 +11,9 @@ To run these samples:
 
 When run, you should see the system-default browser launch and load the
 Dart website, https://dart.dev
+
+## Learn more
+
+To learn more about FFI, start with the [C interop using
+dart:ffi](https://dart.dev/guides/libraries/c-interop) guide on dart.dev.
+