Update changelog for v6 txnbuild release (#3420)

txnbuild/CHANGELOG.md

@@ -3,7 +3,7 @@
 All notable changes to this project will be documented in this
 file.  This project adheres to [Semantic Versioning](http://semver.org/).
 
-## Unreleased
+## [v6.0.0](https://github.com/stellar/go/releases/tag/horizonclient-v6.0.0) - 2021-02-22
 
 ### Breaking changes
 
@@ -15,7 +15,10 @@ file.  This project adheres to [Semantic Versioning](http://semver.org/).
     * `VerifyChallengeTxSigners()`
   * The webAuthDomain parameter is verified in the `Read*` and `Verify*` functions if it is contained in the challenge transaction, and is ignored if the challenge transaction was generated by an older implementation that does not support the webAuthDomain.
   * The webAuthDomain parameter is included in challenge transactions generated in the `Build*` function, and the resulting challenge transaction is compatible with SEP-10 v2.1 or greater.
-* Use strings to represent accounts in Operation structs ([#3393](https://github.com/stellar/go/pull/3393))
+* Use strings to represent source accounts in Operation structs ([#3393](https://github.com/stellar/go/pull/3393)) see example below:
+  ```go
+  bumpSequenceOp := txnbuild.BumpSequence{BumpTo: 100, SourceAccount: "GB56OJGSA6VHEUFZDX6AL2YDVG2TS5JDZYQJHDYHBDH7PCD5NIQKLSDO"}
+  ```
 * Remove `TxEnvelope()` functions from `Transaction` and `FeeBumpTransaction` to simplify the API. `ToXDR()` should be used instead of `TxEnvelope()` ([#3377](https://github.com/stellar/go/pull/3377))
 
 ## [v5.0.1](https://github.com/stellar/go/releases/tag/horizonclient-v5.0.1) - 2021-02-16