[llvm-objcopy][MachO] Implement --add-section

Reviewers: alexshap, rupprecht, jhenderson

Reviewed By: alexshap, jhenderson

Subscribers: mgorny, jakehehrlich, abrachet, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66283
diff --git a/llvm/docs/CommandGuide/llvm-objcopy.rst b/llvm/docs/CommandGuide/llvm-objcopy.rst
index 0574a5e..9237d73 100644
--- a/llvm/docs/CommandGuide/llvm-objcopy.rst
+++ b/llvm/docs/CommandGuide/llvm-objcopy.rst
@@ -43,6 +43,9 @@
  starts with ".note". Otherwise, it will have type `SHT_PROGBITS`. Can be
  specified multiple times to add multiple sections.
 
+ For MachO objects, ``<section>`` must be formatted as
+ ``<segment name>,<section name>``.
+
 .. option:: --binary-architecture <arch>, -B
 
  Ignored for compatibility.