Mojo: Update/add README.md files.

mojo/public/c/environment/async_waiter.h

Index: mojo/public/c/environment/async_waiter.h
diff --git a/mojo/public/c/system/async_waiter.h b/mojo/public/c/environment/async_waiter.h
similarity index 89%
rename from mojo/public/c/system/async_waiter.h
rename to mojo/public/c/environment/async_waiter.h
index ab999a3bd508d9223f708c85551c76b75381545b..05689543b5bb7ec87afc11546f956484a6758f96 100644
--- a/mojo/public/c/system/async_waiter.h
+++ b/mojo/public/c/environment/async_waiter.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef MOJO_PUBLIC_C_SYSTEM_ASYNC_WAITER_H_
-#define MOJO_PUBLIC_C_SYSTEM_ASYNC_WAITER_H_
+#ifndef MOJO_PUBLIC_C_ENVIRONMENT_ASYNC_WAITER_H_
+#define MOJO_PUBLIC_C_ENVIRONMENT_ASYNC_WAITER_H_
 
 #include "mojo/public/c/system/core.h"
 
@@ -37,4 +37,4 @@ struct MojoAsyncWaiter {
 }  // extern "C"
 #endif
 
-#endif  // MOJO_PUBLIC_C_SYSTEM_ASYNC_WAITER_H_
+#endif  // MOJO_PUBLIC_C_ENVIRONMENT_ASYNC_WAITER_H_