bip-138 add string content type

bip-0138.md

@@ -325,6 +325,7 @@ remaining bytes are `PADDING`. A `PAYLOAD` MUST contain at least one item (see
 | 0x00   | End of content items; padding follows  |
 | 0x01   | BIP Number (big-endian uint16)         |
 | 0x02   | Vendor-Specific Opaque Tag             |
+| 0x03   | String                                 |
 
 `LENGTH`: variable-length integer representing the length of `DATA` in bytes.
 
@@ -338,6 +339,8 @@ For `TYPE` values defined above:
 - 0x01: `LENGTH` MUST be omitted and `DATA` is a 2-byte big-endian unsigned integer
   representing the BIP number that defines it.
 - 0x02: `DATA` MUST be `LENGTH` bytes of opaque, vendor-specific data.
+- 0x03: `DATA` MUST be empty, and the following `PLAINTEXT` is the string itself,
+  which MUST be valid UTF-8.
 
 For all `TYPE` values except `0x01`, parsers MUST reject `CONTENT` if `LENGTH` exceeds
 the remaining payload bytes.