Add a proposed CG subgroup charter document.

docs/WASI-proposed-CG-subgroup-charter.md

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+# WebAssembly System Interface Subgroup Charter
+
+The System Interface Subgroup is a sub-organization of the
+[WebAssembly Community Group](https://www.w3.org/community/webassembly/) of the W3C.
+As such, it is intended that its charter align with that of the CG. In particular, 
+the sections of the [CG charter](https://webassembly.github.io/cg-charter/) relating to
+[Community and Business Group Process](https://webassembly.github.io/cg-charter/#process),
+[Contribution Mechanics](https://webassembly.github.io/cg-charter/#contrib),
+[Transparency](https://webassembly.github.io/cg-charter/#transparency), and
+[Decision Process](https://webassembly.github.io/cg-charter/#decision) also apply to the Subgroup.
+
+## Goals
+
+The mission of this sugbroup is to provide a forum for pre-standardization
+collaboration on a system interface API for WebAssembly programs.
+
+## Scope
+
+The Subgroup will consider topics related to system interface APIs, including:
+
+- APIs for host filesystems, network stacks, and other resources.
+- APIs for graphics, audio, input devices
+- APIs for encryption, format conversion, and other transformations
+  (particularly where hardware accelleration may be available on some plaforms)
+
+
+## Deliverables
+
+### Specifications
+The Subgroup may produce several kinds of specification-related work output:
+- Creation of new specifications in standards bodies or working
+groups (e.g. Wasm WG or TC39)
+- Creation of new specifications outside of standards bodies
+(e.g. similar to the LLVM object file format documentation in Wasm tool conventions)
+
+### Non-normative reports
+The Subgroup may produce non-normative material such as requirements
+documents, recommendations, and use cases.
+
+### Software
+The Subgroup may produce software related to Wasm system interface APIs (either
+as standalone libraries, tooling, or integration of interface-related
+functionality in existing CG software such as Binaryen or WABT). Capabilities may
+include:
+- Libraries implementing external standard APIs in terms of WebAssembly
+  System Interface APIs
+- Tools for producing code that uses WebAssembly System Interface APIs
+- Tools for implementing WebAssembly APIs
+- Tools for debugging programs using WebAssembly System Interface APIs
+
+## Amendments to this Charter and Chair Selection
+
+This charter may be amended, and Subgroup Chairs may be selected by vote of the full
+WebAssembly Community Group.
+