Document port_first_files

rebar.config.sample

@@ -41,6 +41,9 @@
 %% expression pass. Default is `"c_src/*.c"'
 {port_sources, ["c_src/*.c", {"R14", ["c_src/*.c"]}]}.
 
+%% Port files to compile before the rest.
+{port_first_files, ["c_src/first.c", "c_src/second.c"]}.
+
 %% Port compilation environment variables. See rebar_port_compiler.erl for
 %% more info. Default is `[]'
 {port_envs, []}.