matrix: add rentention/ for hopefully finding it again

matrix/retention/31days.json

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+{
+      "min_lifetime": 86400000,
+      "max_lifetime": 2678400000
+}

matrix/retention/README.md

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+# Room history retention settings / state event `m.room.retention`
+
+* https://brendan.abolivier.bzh/matrix-retention-policies/
+
+Read that, it explains what is actually done etc.
+
+## Notes
+
+* Matrix uses milliseconds, keep that in mind!
+* Everything requires every server involved to have rention support enabled,
+  otherwise the messages go nowhere and are fetchable upon request.
+* 86400000 = 1 day (seen in min lifetimes)
+* Helpful: https://unitchefs.com/milliseconds/days/ / https://unitchefs.com/days/milliseconds/

matrix/retention/week.json

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+{
+      "min_lifetime": 86400000,
+      "max_lifetime": 604800000
+}