[llvm-ar] Document response file support in --help

Also a test for this.

Differential Revision: https://reviews.llvm.org/D63836

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364673 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-ar/llvm-ar.cpp

@@ -78,6 +78,7 @@ OPTIONS:
   --plugin=<string>     - Ignored for compatibility
   --help                - Display available options
   --version             - Display the version of this program
+  @<file>               - read options from <file>
 
 OPERATIONS:
   d - delete [files] from the archive