Add a document to begin collecting pending ABI changes

Documentation/ABI/Pending-ABIv1.md

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+# Pending ABIv1 Changes
+
+This document collects changes which have been made and are anticipated to be
+released in a future ABI/event stream version, but are still pending.
+
+> [!IMPORTANT]
+> This file describes changes which are unreleased, and are subject to change.
+> Do not depend on this version or make assumptions that these changes will
+> remain part of this ABI version.
+
+## Schema version numbers
+
+The schema version can now be specified as a 2- or 3-part version string such as
+`"6.3"`. Integers continue to be supported for backwards-compatibility with the
+existing "version 0" and Xcode 16 Beta schemas.
+
+```diff
+-<version> ::= "version": 0 ; will be incremented as the format changes
++<version> ::= "version": <version-number>
++
++<version-number> ::= 0 | "<integer>.<integer>" | "<integer>.<integer>.<integer>"
+```
+
+## Issue severity and warning issues
+
+Introduced a notion of "severity" for issues, and a new "warning" severity level
+which is lower than the default "error" level. Clients upgrading from v0 should
+begin consulting the severity of recorded issues to determine whether the
+associated test should be marked as failing.
+
+```diff
+ <issue> ::= {
+   "isKnown": <bool>, ; is this a known issue or not?
++  "severity": <issue-severity>, ; the severity of the issue
++  "isFailure": <bool>, ; did this issue cause the associated test to fail?
+   ["sourceLocation": <source-location>,] ; where the issue occurred, if known
+ }
+
++<issue-severity> ::= "warning" | "error"
++  ; additional severities may be added in the future
+```
+
+### See Also
+
+- https://github.com/swiftlang/swift-testing/pull/931