blip-0002: experimental endorsement TLV reservation for update_add_htlc

This commit reserves TLV number 65555 in update_add_htlc for use in
experimentally deploying HTLC endorsement. The endorsement TLV is
intended to be included in the BOLTs (and use a protocol-range TLV
field), but an experimental field will be used for research purposes.

Upgrading to using a protocol-range TLV will be trivial to implement,
simply copying the experimental range value and propagating both TLVs
during the upgrade period.

blip-0002.md

@@ -110,6 +110,15 @@ The following table contains extension tlv fields for the `ping` message:
 |-------|-----------------------------|--------------------------------|
 | 65536 | `tlv_field_name`            | Link to the corresponding bLIP |
 
+#### `update_add_htlc`
+
+The following table contains extension tlv fields for the `update_add_htlc` message:
+
+
+| Type  | Name                        | Link                           |
+|-------|-----------------------------|--------------------------------|
+| 65555 | `endorsed`                  | [Experimental Endorsement](https://github.com/lightning/bolts/pull/1071) |
+
 ## Copyright
 
 This bLIP is licensed under the CC0 license.