Add a HighLevelGoals.md document.

docs/HighLevelGoals.md

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+# WASI High-Level Goals
+
+(In the spirit of [WebAssembly's High-Level Goals](https://github.com/WebAssembly/design/blob/master/HighLevelGoals.md).)
+
+1. Define a portable, modular, runtime-independent, and WebAssembly-native API
+   to serve as a system interface which can be used by WebAssembly code to
+   interact with the outside world, that preserves the essential sandboxed
+   nature of WebAssembly through a [Capability-based] API design.
+2. Specify and implement incrementally:
+    * Start with a Minimum Viable Product (MVP) for the standard, covering I/O
+      streams, filesystems, randomness, clocks, and program startup and
+      shutdown.
+    * Then add additional features, prioritized by feedback and experience.
+3. Supplement API designs with documentation and tests, and, when feasible,
+   reference implementations which can be shared between wasm engines.
+4. Make a great platform:
+    * Work with WebAssembly tool and library authors to help them provide
+      WASI support for their users.
+    * When being WebAssembly-native means the platform isn't directly
+      compatible with existing applications written for other platforms,
+      build tools and libraries to provide compatibility.
+    * Allow the overall API to evolve over time; to make changes to API
+      modules that have been standardized, build implementations of them
+      using libraries on top of new API modules to provide compatibility.
+
+[Capability-based]: https://en.wikipedia.org/wiki/Capability-based_security