Added comment explaining the protobuf_headers target.

BUILD

@@ -181,6 +181,10 @@ cc_library(
     deps = [":protobuf_lite"],
 )
 
+# This provides just the header files for use in projects that need to build
+# shared libraries for dynamic loading. This target is available until Bazel
+# adds native support for such use cases.
+# TODO(keveman): Remove this target once the support gets added to Bazel.
 cc_library(
     name = "protobuf_headers",
     hdrs = glob(["src/**/*.h"]),