Add application-abi describing the current unstable WASI application …

…ABI (#48)

design/application-abi.md

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+WASI Application ABI
+====================
+
+In addition to the APIs defined by the various WASI [modules](modules.md) there
+are also certain expectations that the WASI runtime places on an application
+that wishes to be portable across WASI implementations.
+
+This document describes how a conforming WASI application is expected to behave
+in terms of lifecycle (startup, shutdown, etc) and any exports it is expected to
+include.
+
+Current Unstable ABI
+--------------------
+
+The current WASI unstable ABI specifies only two exports from a WASI
+application:
+
+- `_start` - the program entry point
+- `memory` - linear memory used by the program
+
+The `_start` export must be WebAssembly function and will be used as the program
+entry point.  This is the default name used by `lld` when linking WebAssembly
+modules.  The embedder is expected to call this function once the module is
+instantiated.
+
+Many of the current WASI unstable APIs require sharing of linear memory between
+the application and the embedder.  In order to use any such APIs the WASI module
+is expected to export its linear memory under the name `memory`.
+
+Planned Stable ABI
+------------------
+
+There is ongoing discussion about what the stable ABI might look like:
+
+- https://github.com/WebAssembly/WASI/issues/13
+- https://github.com/WebAssembly/WASI/issues/19
+- https://github.com/WebAssembly/WASI/issues/24