Add some documentation to FindNanopb.cmake about the new RELPATH option.

extra/FindNanopb.cmake

@@ -28,11 +28,16 @@
 #  ====================================================================
 #
 # NANOPB_GENERATE_CPP (public function)
-#   SRCS = Variable to define with autogenerated
-#          source files
-#   HDRS = Variable to define with autogenerated
-#          header files
-#   ARGN = proto files
+# NANOPB_GENERTAE_CPP(SRCS HDRS [RELPATH <root-path-of-proto-files>] 
+#                     <proto-files>...)
+#   SRCS = Variable to define with autogenerated source files
+#   HDRS = Variable to define with autogenerated header files
+#   If you want to use relative paths in your import statements use the RELPATH
+#   option. The argument to RELPATH should be the directory that all the
+#   imports will be relative to.
+#   When RELPATH is not specified then all proto files can be imported without
+#   a path. 
+#
 #
 #  ====================================================================
 #  Example:
@@ -47,6 +52,19 @@
 #   include_directories(${CMAKE_CURRENT_BINARY_DIR})
 #   add_executable(bar bar.cc ${PROTO_SRCS} ${PROTO_HDRS})
 #
+#  Example with RELPATH:
+#   Assume we have a layout like:
+#    .../CMakeLists.txt
+#    .../bar.cc
+#    .../proto/
+#    .../proto/foo.proto  (Which contains: import "sub/bar.proto"; )
+#    .../proto/sub/bar.proto
+#   Everything would be the same as the previous example, but the call to
+#   NANOPB_GENERATE_CPP would change to:
+# 
+#   NANOPB_GENERATE_CPP(PROTO_SRCS PROTO_HDRS RELPATH proto
+#                       proto/foo.proto proto/sub/bar.proto)
+# 
 #  ====================================================================
 
 #=============================================================================