Clarify that Baseline does not cover assistive technology (#519)

This adds an item to non-goals to clarify that support in assistive tools that
may be used along a browser is not (yet?) covered by Baseline.

See discussion in #498

Co-authored-by: Daniel D. Beck <daniel@ddbeck.com>
Co-authored-by: Philip Jägenstedt <philip@foolip.org>

docs/baseline.md

@@ -59,6 +59,9 @@ Baseline status cannot or will not satisfy the following non-goals:
   Many Baseline features will not achieve 100% user reach soon or perhaps ever.
   If a web developer needs to support a globally uncommon or discontinued browser (e.g., Internet Explorer 11), then the developer needs to know the specific limitations of that browser, not a broad overview of developers’ most commonly required browsers.
   Baseline can’t be both.
+* **Identify support in assistive technology.**
+  Baseline does not cover support for screen readers, screen magnifiers, voice control, and other assistive technology that is not built into browsers.
+  See also: [Future considerations](#future-considerations).
 * **Identify support in non-web environments.**
   Developers use web technologies in non-web settings, such as JavaScript in Node.js, Web APIs in Deno, or HTML and CSS in Electron-based applications.
   For good reasons, web technologies in non-web settings often depart from interoperability with web browsers.
@@ -176,5 +179,6 @@ Here are some areas for future consideration and not currently in-scope for Base
 * Progressive enhancement safe (i.e., limited penalties for support failures)
 * Developer feedback requested
 * Buggy (e.g., supported but in ways that are surprising and semi-interoperable)
+* Support in assistive technology that is not built into browsers.
 * Obsolete/deprecated/legacy/etc. (i.e., flagged as such in the specification or dropped from newer versions of the specification)
 * Having high-quality polyfills available