Add a reference for Elf_Chdr type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274793 91177308-0d34-0410-b5e6-96231b3b80d8

Author: rui314

include/llvm/Object/ELFTypes.h

@@ -561,6 +561,7 @@ struct Elf_GnuHash_Impl {
 };
 
 // Compressed section headers.
+// http://www.sco.com/developers/gabi/latest/ch4.sheader.html#compression_header
 template <endianness TargetEndianness>
 struct Elf_Chdr_Impl<ELFType<TargetEndianness, false>> {
   LLVM_ELF_IMPORT_TYPES(TargetEndianness, false)