Update changelog

CHANGES.md

@@ -31,6 +31,12 @@ To be released.
     [[#204]], [[#206]]
  -  Added `BlockDownloadState` class to represent a block downloading state.
     [[#204]], [[#206]]
+ -  The type of`Block<T>.Difficulty` is changed to `long` instead of `int`, and
+    related classes method parameters and field types have changed accordingly.
+ -  Removed `HashDigest.HasLeadingZeroBits()` method.  [[#213]]
+ -  Added `HashDigest.LessThanTarget()` method.  [[#213]]
+ -  `BlockPolicy<T>` constructor became to receive the minimum difficulty and
+    the mining difficulty bound divisor.  [[#213]]
  -  Improved overall read throughput of `BlockChain<T>` while blocks are being
     mined by `BlockChain<T>.MineBlock()`.
  -  Fixed a bug that `TurnClientException` had been thrown by Swarm when a STUN
@@ -49,6 +55,8 @@ To be released.
     minutes.
  -  Instead of validating the entire blocks, `BlockChain<T>.Append()` method
     became to validate only the next block to be appended.  [[#210]]
+ -  The calculation algorithm of `BlockPolicy<T>.GetNextBlockDifficulty()`
+    method is changed.  [[#213]]
 
 [#185]: https://github.com/planetarium/libplanet/pull/185
 [#187]: https://github.com/planetarium/libplanet/issues/187
@@ -58,6 +66,7 @@ To be released.
 [#205]: https://github.com/planetarium/libplanet/pull/205
 [#206]: https://github.com/planetarium/libplanet/pull/206
 [#210]: https://github.com/planetarium/libplanet/pull/210
+[#213]: https://github.com/planetarium/libplanet/pull/213
 
 
 Version 0.2.2