doc: Describe reverse adaptor sig protocol.

doc/protocols/secret_hash_seller_first.md

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+# Seller first protocol
+
+Seller sends the first transaction.
+Both coin types must support scripts.
+
+
+1. Seller posts offer.
+    - smsg from seller to network
+        coin-from
+        coin-to
+        amount-from
+        rate
+        min-amount
+        time-valid
+
+2. Buyer posts bid:
+    - smsg from buyer to seller
+        offerid
+        amount
+        proof-of-funds
+        address_to_buyer
+        time-valid
+
+3. Seller accepts bid:
+    - verifies proof-of-funds
+    - generates secret
+    - submits initiate tx to coin-from network
+    - smsg from seller to buyer
+        txid
+        initiatescript (includes pkhash_to_seller as the pkhash_refund)
+
+4. Buyer participates:
+    - inspects initiate tx in coin-from network
+    - submits participate tx in coin-to network
+
+5. Seller redeems:
+    - constructs participate script
+    - inspects participate tx in coin-to network
+    - redeems from participate tx revealing secret
+
+6. Buyer redeems:
+    - scans coin-to network for seller-redeem tx
+    - redeems from initiate tx with revealed secret