Add zephyr to README integrations

Add an entry for the nanopb integration in Zephyr with a link to
the documentation.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>
diff --git a/README.md b/README.md
index 629ae72..38e94a3 100644
--- a/README.md
+++ b/README.md
@@ -96,4 +96,5 @@
 And also integration to platform interfaces:
 
 * **Arduino**: http://platformio.org/lib/show/1385/nanopb-arduino
+* **Zephyr**: https://docs.zephyrproject.org/latest/services/serialization/nanopb.html