Add a CHANGELOG file

Add a CHANGELOG file including release notes for the current
version (minor and point) as well as an "unrleased" section for the
upcoming release.

Author: tcharding

CHANGELOG.md

@@ -0,0 +1,13 @@
+# unreleased (0.10.0)
+
+- [Implement `std::error::Error::source` for error type](https://github.com/rust-bitcoin/rust-bech32/pull/72)
+
+# 0.9.1
+
+<!-- Woops, added to the API in a point release -->
+- [Support bech32 encoding without a checksum](https://github.com/rust-bitcoin/rust-bech32/pull/66)
+
+# 0.9.0
+
+- [Enable edition 2018](https://github.com/rust-bitcoin/rust-bech32/pull/57) bumping MSRV to 1.41.1
+- [Implement `From<u5> for u8`](https://github.com/rust-bitcoin/rust-bech32/pull/58)