Update the changelog and the migration guide (#1016)

Author: dmitry-timofeev

exonum-java-binding/CHANGELOG.md

@@ -19,7 +19,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Overview
 
-This release brings support of Exonum TestKit. It is based on Exonum 0.11.
+This release brings support of Exonum TestKit, massive performance improvements,
+and various other fixes and improvements. It is based on Exonum 0.11.
+
+*If you are upgrading an existing Java service, consult 
+the [migration guide](https://github.com/exonum/exonum-java-binding/blob/ejb/v0.7.0/exonum-java-binding/doc/Migration_guide_0.7.md).*
 
 ### Added
 - A new `exonum-testkit` module that allows to emulate blockchain network and test transaction
@@ -39,6 +43,9 @@ This release brings support of Exonum TestKit. It is based on Exonum 0.11.
 [testkit-documentation]: https://exonum.com/doc/version/0.11/get-started/java-binding/#testing
 
 ### Changed
+- Improved the throughput of transaction processing twofold. Java services on Exonum Java 0.7.0 
+  handle transactions about 15–20% slower than equivalent Rust ones, according to our system 
+  benchmarks. (#917, #996)
 - `BinaryTransactionMessage#toString` to include some fields in human-readable
   format instead of the whole message in binary form.
 - `Node#submitTransaction` to throw _unchecked_ `TransactionSubmissionException` instead

exonum-java-binding/doc/Migration_guide_0.7.md

@@ -13,6 +13,14 @@ Find: (com.exonum.binding)(.)(annotations|blockchain|proxy|runtime|service|stora
 Replace: $1$2core$2$3
 ```
 
-<!-- TODO: ## Explore the New Features --> 
+## Explore the New Features
+
+0.7.0 brings Exonum Testkit, which allows to test service operations in an emulated blockchain
+network. See the [documentation][testkit-documentation] for more information and examples.
+
+## See Also
+
+The 0.7.0 [release page][release-page] for the changelog and pre-built binaries.
 
 [release-page]: https://github.com/exonum/exonum-java-binding/releases/tag/ejb/v0.7.0
+[testkit-documentation]: https://exonum.com/doc/version/0.11/get-started/java-binding/#testing