Mojo: Update/add README.md files.

mojo/mojo_public.gypi

 {
   'targets': [
     {
       'target_name': 'mojo_system',
       'type': 'static_library',
       'defines': [
         'MOJO_SYSTEM_IMPLEMENTATION',
       ],
       'include_dirs': [
         '..',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '..',
         ],
       },
       'all_dependent_settings': {
         'conditions': [
           # We need to be able to call the MojoSetSystemThunks() function in
           # system_thunks.cc
           ['OS=="android"', {
             'ldflags!': [
               '-Wl,--exclude-libs=ALL',
             ],
           }],
         ],
       },
       'sources': [
-        'public/c/system/async_waiter.h',
         'public/c/system/core.h',
         'public/c/system/macros.h',
         'public/c/system/system_export.h',
         'public/platform/native/system_thunks.cc',
         'public/platform/native/system_thunks.h',
       ],
     },
     {
       'target_name': 'mojo_gles2',
       'type': 'shared_library',
@@ skipping 315 matching lines @@
       ],
       'dependencies': [
         'mojo_shell_bindings',
       ],
       'export_dependent_settings': [
         'mojo_shell_bindings',
       ],
     },
   ],
 }