doc: add maintaining-webassembly.md

Add maintaining-webassembly.md with strategy based
on discussion in Next-10 mini-summit:
nodejs/next-10#127

Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: nodejs/node#42660
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/contributing/maintaining-web-assembly.md

@@ -0,0 +1,97 @@
+# Maintaining WebAssembly
+
+Support for [WebAssembly](https://webassembly.org/)
+has been identified as one of the
+[top technical priorities](https://github.com/nodejs/node/blob/master/doc/contributing/technical-priorities.md#webassembly)
+for the future success of Node.js.
+
+This document provides an overview of our high-level strategy for
+supporting WebAssembly and information about our current implementation
+as a starting point for contributors.
+
+## High-level approach
+
+The key elements of our WebAssembly strategy include:
+
+* Up-to-date core WebAssembly support
+* Support for high-level APIs
+* Making it easy to load WebAssembly
+* Making sure the core Node.js APIs are compatible with WebAssembly
+  and can be called in an efficient manner from WebAssembly
+
+### Up-to-date core WebAssembly support
+
+Node.js gets its core WebAssembly support through V8. We don't need
+to do anything specific to support this, all we have to do is keep
+the version of V8 as up-to-date as possible.
+
+### Key API support
+
+As a runtime, Node.js must implement a number of APIs in addition
+to the core WebAssembly support in order to be a good choice to run
+WebAssembly. The project has currently identified these additional
+APIs as important:
+
+* WebAssembly System Interface (WASI). This provides the ability for
+  WebAssembly to interact with the outside world. Node.js currently
+  has an implementation (see below for more details).
+* WebAssembly streaming APIs - As the Node.js implementation of
+  [WebStreams](https://nodejs.org/api/webstreams.html) matures,
+  implementing the embedder APIs to enable streaming with WebAssembly
+  will be important.
+* [WebAssembly Component Model](https://github.com/WebAssembly/component-model/).
+  This API is still in the definition stage but the project should
+  keep track of its development as a way to simplify native code
+  integration.
+
+### Making it as easy as possible to load WASM
+
+The most important thing we can do on this front is to either find and
+reference resources or provide resources on how to:
+
+* Compile your WebAssembly code (outside of Node.js) and integrate that
+  into an npm workflow.
+* Load and run WebAssembly code in your Node.js application.
+
+It is also important to support and track the ongoing work in ESM to enable
+loading of WebAssembly with ESM.
+
+### Making sure the core Node.js APIs are compatible with WebAssembly
+
+Use cases for which Node.js will be a good runtime will include code
+both in JavaScript and compiled into WebAssembly. It is important
+that Node.js APIs are able to be called from WebAssembly in
+an efficient manner without extra buffer copies. We need to:
+
+* Review APIs and identify those that can be called often from
+  WebAssembly.
+* Where appropriate, make additions to identified APIs to allow
+  a pre-existing buffer to be passed in order to avoid copies.
+
+## Current implementation and assets
+
+### WebAssembly System Interface (WASI)
+
+The Node.js WASI implementation is maintained in the
+[uvwasi](https://github.com/nodejs/uvwasi) repository in the
+Node.js GitHub organization. As needed, an updated copy
+is vendored into the Node.js deps in
+[deps/uvwasi](https://github.com/nodejs/node/tree/master/deps/uvwasi).
+
+To update the copy of uvwasi in the Node.js deps:
+
+* Copy over the contents of `include` and `src` to the corresponding
+  directories.
+* Check if any additional files have been added and need to be added
+  to the `sources` list in `deps/uvwasi/uvwasi.gyp`.
+
+In addition to the code from uvwasi, Node.js includes bindings and
+APIs that allow WebAssembly to be run with WASI support from Node.js.
+The documentation for this API is in
+[WebAssembly System Interface (WASI)](https://nodejs.org/api/wasi.html).
+
+The implementation of the bindings and the public API is in:
+
+* [src/node\_wasi.h](https://github.com/nodejs/node/blob/master/src/node_wasi.h)
+* [src/node\_wasi.cc](https://github.com/nodejs/node/blob/master/src/node_wasi.cc)
+* [lib/wasi.js](https://github.com/nodejs/node/blob/master/lib/wasi.js)