443: add paragraph on fee management

Author: bigspider

bip-0443.mediawiki

@@ -129,6 +129,18 @@ Note that the ''deduct'' semantic does not allow to check the exact amount of it
 a scheme similar to figure 3 or 4 above, amounts should be constrained either with a signature, or with future
 introspection opcodes that allow fixing the amount. In lack of that, amounts would be malleable.
 
+=== Paying fees ===
+
+Since the amount-checking semantics of <code>OP_CHECKCONTRACTVERIFY</code> are designed to preserve the entire input
+amount across one or more outputs, transaction fees must be paid exogenously. This can be achieved by adding an extra
+input to the transaction, by using an anchor output, or with other future mechanisms.
+
+The ''ignore'' amount mode is not a safe mechanism for paying endogenous fees. An output checked with this mode has no
+amount constraint, which would allow a miner to claim the entire value of that input. This mode is included for forward
+compatibility with potential future soft forks that may introduce other amount-related logic that is compatible with
+<code>OP_CHECKCONTRACTVERIFY</code>'s script checks.
+
+
 == Specification ==
 
 The tapscript opcode <code>OP_SUCCESS187</code> (<code>0xbb</code>) is constrained with new rules to implement