Spec MSC2285: Private read receipts (#1216)

* Convert `m.receipt.yaml` to traditional YAML

* Spec MSC2285 (private read receipts)

* Add some obvious copyright headers

* Add changelog entries

* Appease the linter

Apparently it hates it when you do this.

* Allow m.fully_read on /receipts

* Apply suggestions from code review

Co-authored-by: Matthew Hodgson <matthew@matrix.org>

Co-authored-by: Matthew Hodgson <matthew@matrix.org>

data/event-schemas/examples/m.receipt.yaml

@@ -7,6 +7,11 @@
         "@rikj:jki.re": {
           "ts": 1436451550453
         }
+      },
+      "m.read.private": {
+        "@self:example.org": {
+          "ts": 1661384801651
+        }
       }
     }
   }