message: chore: amend CHANGELOG for v0.2.0
diff: @@ -18,6 +18,9 @@ In this release, we have upgraded to Rust 2018. We also did 2 important refactor
  * fix syscall length calculation ([#82](https://github.com/nervosnetwork/ckb/issues/82)) ([fb23f33](https://github.com/nervosnetwork/ckb/commit/fb23f33))
  * in case of missing cell, return `ITEM_MISSING` error instead of halting ([707d661](https://github.com/nervosnetwork/ckb/commit/707d661))
  * remove hash caches to avoid JSON deserialization bug ([#84](https://github.com/nervosnetwork/ckb/issues/84)) ([1274b03](https://github.com/nervosnetwork/ckb/commit/1274b03))
  +* fix `rpc_url` ([62e784f](https://github.com/nervosnetwork/ckb/commit/62e784f))
  +* resolve mining old block issue ([#87](https://github.com/nervosnetwork/ckb/issues/87)) ([01e02e2](https://github.com/nervosnetwork/ckb/commit/01e02e2))
  +* uncheck subtract overflow ([#88](https://github.com/nervosnetwork/ckb/issues/88)) ([2b0976f](https://github.com/nervosnetwork/ckb/commit/2b0976f))
  ### Features
  