cpp11-migrate: Docs refresh

* Documented new command-line options.
* Moved usage to a new page.
  * Usage now split into general options and transform-related options.
* Main Migrator page now contains getting started and getting involved
  information.
  * Also included a JIRA issue collector button for logging bugs.

llvm-svn: 183417
diff --git a/clang-tools-extra/docs/AddOverrideTransform.rst b/clang-tools-extra/docs/AddOverrideTransform.rst
index ea537fe..03f9494 100644
--- a/clang-tools-extra/docs/AddOverrideTransform.rst
+++ b/clang-tools-extra/docs/AddOverrideTransform.rst
@@ -25,6 +25,16 @@
     void h() const override;
   };
 
+Using Expands-to-Override Macros
+--------------------------------
+
+Like LLVM's ``LLVM_OVERRIDE``, several projects have macros that conditionally
+expand to the ``override`` keyword when compiling with C++11 features enabled.
+To maintain compatibility with non-C++11 builds, the Add-Override Transform
+supports detection and use of these macros instead of using the ``override``
+keyword directly. Specify ``-override-macros`` on the command line to the
+Migrator to enable this behavior.
+
 
 Known Limitations
 -----------------