Add opt-in documentation for tx retention feature

CHANGELOG.md

@@ -1,7 +1,11 @@
-# Changelog 
+# Changelog
 
 ## Unreleased
-n/a
+
+### Added
+* Transactions will now be stored in a database rather than in memory ([#174](https://github.com/stellar/soroban-rpc/pull/174)).
+
+You can opt-in to longer transaction retention by setting `--transaction-retention-window` / `TRANSACTION_RETENTION_WINDOW` to a higher number of ledgers. This will also retain corresponding number of ledgers in the database. Keep in mind, of course, that this will cause an increase in disk usage for the growing database.
 
 
 ## [v21.1.0](https://github.com/stellar/soroban-rpc/compare/v21.0.1...v21.1.0)
@@ -20,10 +24,10 @@ interface getVersionInfo {
 ```
 
 ### Fixed
-* Deadlock on events ingestion error ([#167](https://github.com/stellar/soroban-rpc/pull/167)). 
-* Correctly report row iteration errors in `StreamAllLedgers` ([#168](https://github.com/stellar/soroban-rpc/pull/168)). 
-* Increase default ingestion timeout ([#169](https://github.com/stellar/soroban-rpc/pull/169)). 
-* Surface an ignored error in `getRawLedgerEntries()` ([#170](https://github.com/stellar/soroban-rpc/pull/170)). 
+* Deadlock on events ingestion error ([#167](https://github.com/stellar/soroban-rpc/pull/167)).
+* Correctly report row iteration errors in `StreamAllLedgers` ([#168](https://github.com/stellar/soroban-rpc/pull/168)).
+* Increase default ingestion timeout ([#169](https://github.com/stellar/soroban-rpc/pull/169)).
+* Surface an ignored error in `getRawLedgerEntries()` ([#170](https://github.com/stellar/soroban-rpc/pull/170)).
 
 
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