docs(changelog): the 0.4.1 -> 0.6.1 gap statement, as a grouped fragment

Three Changed entries naming what a reader comparing the two forges' tags
would otherwise reconstruct: 0.5.0/0.6.0 arrived by merge and were never
released here; the carried ## 0.6.0 section is upstream's record; and the
absent 0.6.0 tag is a decision, not an omission. Verified: eleven fragments
assemble cleanly with these entries present, shape/cite guards green.

Refs #220

changelog.d/220.md

@@ -0,0 +1,13 @@
+### Changed
+
+- This forge's release line runs `0.4.1 → 0.6.1`: versions 0.5.0 and 0.6.0
+  arrived here by merge from the read-only upstream and were never released
+  on this forge (#220).
+
+- The `## 0.6.0` section this changelog carries is upstream's — its entries
+  describe upstream's work under upstream's issue numbers. The forge port's
+  own work ships first in 0.6.1 (#220).
+
+- No `0.6.0` tag exists here, by decision rather than omission: tagging it
+  would publish notes describing work this forge did not do while omitting
+  every issue it did (#220, #219).