Add pointer spec (#2383)

docs/src/inscriptions/pointer.md

@@ -0,0 +1,64 @@
+Pointer
+=======
+
+In order to make an inscription on a sat other than the first of its input, a
+zero-based integer, called the "pointer", can be provided with tag `2`, causing
+the inscription to be made on the sat at the given position in the outputs. If
+the pointer is equal to or greater than the number of total sats in the outputs
+of the inscribe transaction, it is ignored, and the inscription is made as
+usual. The value of the pointer field is a little endian integer, with trailing
+zeroes ignored.
+
+An even tag is used, so that old versions of `ord` consider the inscription to
+be unbound, instead of assigning it, incorrectly, to the first sat.
+
+This can be used to create multiple inscriptions in a single transaction on
+different sats, when otherwise they would be made on the same sat.
+
+Examples
+--------
+
+An inscription with pointer 255:
+
+```
+OP_FALSE
+OP_IF
+  OP_PUSH "ord"
+  OP_PUSH 1
+  OP_PUSH "text/plain;charset=utf-8"
+  OP_PUSH 2
+  OP_PUSH 0xff
+  OP_PUSH 0
+  OP_PUSH "Hello, world!"
+OP_ENDIF
+```
+
+An inscription with pointer 256:
+
+```
+OP_FALSE
+OP_IF
+  OP_PUSH "ord"
+  OP_PUSH 1
+  OP_PUSH "text/plain;charset=utf-8"
+  OP_PUSH 2
+  OP_PUSH 0x0001
+  OP_PUSH 0
+  OP_PUSH "Hello, world!"
+OP_ENDIF
+```
+
+An inscription with pointer 256, with trailing zeroes, which are ignored:
+
+```
+OP_FALSE
+OP_IF
+  OP_PUSH "ord"
+  OP_PUSH 1
+  OP_PUSH "text/plain;charset=utf-8"
+  OP_PUSH 2
+  OP_PUSH 0x000100
+  OP_PUSH 0
+  OP_PUSH "Hello, world!"
+OP_ENDIF
+```