CH08: add conclusion

chapters/signatures.adoc

@@ -1000,3 +1000,15 @@ The new algorithm allows the number of
 hash operations increases by a much more gradual O(n) to the number of
 signature operations, reducing the opportunity to create
 denial-of-service attacks with overly complex transactions.
+
+In this chapter, we learned about schnorr and ECDSA signatures for
+Bitcoin.  This explains how full nodes authenticate transactions to
+ensure that only someone controlling the key to which bitcoins were
+received can spend those bitcoins.  We also examined several advanced
+applications of signatures, such as scriptless multisignatures and
+scriptless threshold signatures that can be used to improve the
+efficiency and privacy of Bitcoin.  In the past few chapters, we've
+learned how to create transactions, how to secure them with
+authorization and authentication, and how to sign them.  We will next
+learn how to encourage miners to confirm them by adding fees to the
+transactions we create.