FindNanopb.cmake: Add PROTOC_OPTIONS documentation (#771)

extra/FindNanopb.cmake

diff --git a/extra/FindNanopb.cmake b/extra/FindNanopb.cmake
index b2f1c2d..60a4f95 100644
--- a/extra/FindNanopb.cmake
+++ b/extra/FindNanopb.cmake
@@ -25,6 +25,7 @@
 #                                     under build directory, instead of mirroring
 #                                     relative paths of source directories.
 #                                     Set to FALSE if you want to disable this behaviour.
+#   PROTOC_OPTIONS           - Pass options to protoc executable 
 #
 # Defines the following variables:
 #