README: add NIP-73 k tag

README.md

@@ -256,6 +256,7 @@ They exist to document what may be implemented by [Nostr](https://github.com/nos
 | `h` | group id | -- | [29](29.md) |
 | `i` | external identity | proof, url hint | [39](39.md), [73](73.md) |
 | `k` | kind number (string) | -- | [18](18.md), [25](25.md), [72](72.md) |
+| `k` | external content id kind | -- | [73](73.md) |
 | `l` | label, label namespace | -- | [32](32.md) |
 | `L` | label namespace | -- | [32](32.md) |
 | `m` | MIME type | -- | [94](94.md) |