doc: add policy for distribution

doc/contributing/distribution.md

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+# Node.js Distribution Policy
+
+This document describes some policies around what is and is not included in the
+Node.js distribution.
+
+## Inclusion
+
+The Node.js distribution includes some external software that the Node.js
+project does not maintain. The choice to include a particular piece of software
+should not imply anything about that software relative to its competitors; in
+some cases, software was added when it had no competitors. While the Node.js
+project supports and encourages competition in the JavaScript ecosystem, as a
+policy, the Node.js project does not include multiple dependencies or tools that
+serve the same purpose.
+
+The following user-accessible external tools or libraries are the ones chosen
+for their particular purposes:
+
+* JavaScript engine: V8
+* Package manager: `npm`
+* Package manager version manager: Corepack
+
+Being user-accessible, removal or replacement of these projects could happen
+only as a semver-major change, unless the related feature or project is
+documented as experimental. In addition, Node.js includes external projects as
+internal dependencies. These may be replaced or removed at any time, provided
+that doing so is not a breaking change.