Add XLS-60d: 0060 XLS-60d: DefaultAutoBridge (#348)

* Add XLS-60d: 0060 XLS-60d: DefaultAutoBridge

* Update XLS-0060-default-autobridge/README.md

* Update XLS-0060-default-autobridge/README.md

---------

Co-authored-by: Vito Tumas <5780819+Tapanito@users.noreply.github.com>

Author: mvadari

XLS-0060-default-autobridge/README.md

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+<pre>
+  xls: 60
+  title: Default AutoBridge
+  description: Use autobridging in IOU-IOU Payment transactions
+  author: tequ (@tequdev)
+  created: 2024-02-05
+  status: Stagnant
+  category: Amendment
+</pre>
+
+## Abstract
+
+Currently, if the Path field is not specified in an IOU-IOU Payment transaction, only direct pairs of two currencies are used internally. This proposal would change this default behavior to use IOU-XRP-IOU as well as OfferCreate transaction's.
+
+## Changes
+
+If the Path field is unspecified in the IOU_A->IOU_B Payment transaction, the path through IOU_A/XRP and IOU_B/XRP is used in addition to the IOU_A/IOU_B path.
+
+If existing transaction types or future transaction types to be implemented are similarly cross-currency transactions, it will be possible to use bridge paths in addition to direct paths by default.
+
+### Technical
+
+Add an additional path with XRP as the intermediate between two currencies to in flow() method in Flow.cpp when `defaultPaths`=`true`.
+
+Use XRP-mediated paths to the paths under the following conditions:
+
+- Path is not specified
+- SendMax is set.
+- The issuer and currency of Amount and SendMax are different.
+
+### Payment transaction
+
+If the Path field is not specified and the SendMax field is set, the transaction will use the XRP-mediated path.
+
+No impact if the NoRippleDirect flag is set.
+
+### OfferCreate transaction
+
+No impact as Path is always set.
+
+### CheckCash transaction
+
+(using flow())
+
+No impact as it only processes when the fields corresponding to Amount and SendMax are the same.
+
+_If future amendments allow cross-currency checks, composite paths will be available._
+
+### XChainBridge transaction
+
+(using flow())
+
+No impact as SendMax is processed as null.
+
+### PathFind command
+
+(using flow())
+No impact as DefaultPaths is false.