docs(json-crdt): ✏️ add indexed codec README

Author: streamich

src/json-crdt/codec/indexed/binary/README.md

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+## Binary Indexed Format
+
+The Binary Index encoding format for JSON CRDT returns a flat map, where each
+key is a string and each value is an `Uint8Array` blob.
+
+The map has the following keys:
+
+- `"r"` - ID of the root node.
+- `"c"` - Clock table of the document.
+- `"{sid}_{time}` - Each key is a string of the form `{sid}_{time}`, where
+  `sid` is the session ID index in the clock table encoded as Base36 and `time`
+  is the logical clock sequence number encoded as Base36. The value is the
+  encoded node.