Update FindNanopb.cmake comments (#527)
diff --git a/extra/FindNanopb.cmake b/extra/FindNanopb.cmake
index c696d4d..ee7a6fc 100644
--- a/extra/FindNanopb.cmake
+++ b/extra/FindNanopb.cmake
@@ -21,6 +21,9 @@
#
# NANOPB_GENERATE_CPP_APPEND_PATH - By default -I will be passed to protoc
# for each directory where a proto file is referenced.
+# This causes all output files to go directly
+# under build directory, instead of mirroring
+# relative paths of source directories.
# Set to FALSE if you want to disable this behaviour.
#
# Defines the following variables: