Add browser support policy for Scribble-generated documentation.

scribble-lib/scribble/browser-support.md

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+## Scribble Browser Support Policy
+
+This document describes the intended browser support range for HTML
+manuals generated with `#lang scribble/manual`.
+
+This policy defines three levels of support:
+
+- Full support (all features). 
+- Partial support (documentation features)
+- Minimal support (reading documentation text)
+
+Any feature must work correctly in all browsers with full support.
+
+Features that are not needed for reading and understanding the
+documentation, but that are intended for developers of the
+documentation or other specialized audiences, such as links to
+implementation, may not work in browsers with only partial support.
+
+For browsers with minimal support, it will be possible to read the
+text of the documentation, but all other features (such as proper
+styling or links) may be missing.
+
+#### Currently supported browsers
+
+- All "evergreen" browsers current within the past 12 months (this
+  includes Chrome, Firefox, Safari, Edge, and other regularly-updated
+  browsers based on similar). "Current" means if that browser was
+  shipped to users within the past 12 months, it is fully supported.
+
+- Partial support includes any browser released within the past 5
+  years with at least 1% use on https://docs.racket-lang.org As of
+  June 1, 2020, that includes the above browsers only.
+
+  Note that the only browser not in the above set with more than 1%
+  share is Chrome 38 as distributed on the Nexus 5, which was
+  discontinued in March 2015.
+
+- Minimal support applies to all widely-distributed browsers. That is,
+  it is important that the text of the documentation appears in Links,
+  IE 6, Opera Mini, etc.
+