units: Add an initial changelog

In preparation for the initial release add a changelog. Note the version
number is already set to `v0.1.0` and this does not conflict with the
release currently on crates.io `v0.0.0`.

units/CHANGELOG.md

@@ -0,0 +1,17 @@
+# 0.1.0 - Initial Release - 2024-04-03
+
+Initial release of the `bitcoin-units` crate. These unit types are
+integer wrapper types used by the `rust-bitcoin` ecosystem. Note
+please that this release relies heavily on the "alloc" feature.
+
+The main types are:
+
+- `Amount`
+- `locktime::absolute::{Height, Time}`
+- `locktime::relative::{Height, Time}`
+- `FeeRate`
+- `Weight`
+
+# 0.0.0 - Placeholder release
+
+Empty crate to reserve the name on crates.io