Add a changelog

CHANGELOG.md

+
+# 0.4.1 (unreleased)
+
+* Expose `buf` module and have most types available from there vs. root.
+* Implement `IntoBuf` for `T: Buf`.
+* Add `FromBuf` and `Buf::collect`.
+* Add iterator adapter for `Buf`.
+* Add scatter/gather support to `Buf` and `BufMut`.
+* Add `Buf::chain`.
+* Reduce allocations on repeated calls to `BytesMut::reserve`.
+* Implement `Debug` for more types.
+* Remove `Source` in favor of `IntoBuf`.
+* Implement `Extend` for `BytesMut`.
+
+
+# 0.4.0 (February 24, 2017)
+
+* Initial release