[lightningnetwork/lightning-rfc] Max capacity in channel update (#465)

cdecker commented on this pull request.

> @@ -382,6 +384,19 @@ individual bits:
| 0 | `direction` | Direction this update refers to. |
| 1 | `disable` | Disable the channel. |

+The `message_flags` bitfield is used to indicate the presence of optional
+fields in the `channel_update` message:
+
+| Bit Position | Name | Field |
+| ————- | ————————- | ——————————— |
+| 0 | `option_channel_htlc_max` | `htlc_maximum_msat` |

I’d rather have this in the `channel_announcement`, maybe we can use the features bitfield to signal this? Anyway I expect everybody will just add this in perpetuity so a bit is not really needed.