[ffumero2003]

Context

A pre-1.0 minor release can break callers, but the only record is the full CHANGELOG. Users whose build or a call breaks have to read the whole list to find why. Closes #314.

Changes

• Add docs/upgrading.md: one section per breaking release listing only what callers must change, with before/after code, assembled from the **Breaking:** changelog entries. CHANGELOG.md stays the full record.
• Covers v0.10.0 — twelve breaking changes, split into the two that require a source change (WriterOption → WriterOptionFunc; the variadic keys ...*SSECKey on NewReader/NewRangeReader/Attrs) and the rest, which fail closed on specific call patterns (CopyToBucket(nil), empty BucketIDs, invalid lifetimes, ranged-response validation, replication on create/update, sync default compare + platform support, NewRangeReader length 0, multiple FileLock, Writer.Close waiting).
• Backfills v0.9.0 (Go 1.26.10 floor, base.CreateBucket new args, upload retry log text).
• Adds the docs/README.md index row.

Backfill range: v0.9.0 and v0.10.0 (the releases with **Breaking:** entries).

Verification

• git diff --check: clean.
• Every after snippet was compiled against the v0.10.0 API (current main) in a temporary in-module package — go build ./... succeeds: WriterOptionFunc, the NewReader closure, Copy with CopyToBucket, CreateKey with BucketIDs and with Lifetime, NewBucket with attrs.ReplicationConfig = nil, Sync with WithCompare(CompareSHA1), and NewRangeReader(ctx, offset, -1). The temporary file was removed; the PR contains only the two docs.
• Cross-checked each entry against the corresponding **Breaking:** line in .changes/v0.10.0.md.

Docs-only; no library behavior change (uses skip-changelog).

Closes #314