docs: add changelog

.clog.toml

+[clog]
+repository = "https://github.com/zkat/big-brain"
+subtitle = ""
+changelog = "CHANGELOG.md"
+from-latest-tag = true

CHANGELOG.md

+<a name="0.3.0"></a>
+## 0.3.0 (2021-04-14)
+
+This is a major overhaul of the Bevy API. It removes `.ron` support
+altogether, in favor of plain old Rust builders.
+
+#### Breaking Changes
+
+* The `.ron` Thinker definition API is gone. Use the ThinkerBuilder API instead.
+* The `Action` and `Scorer` derive macros are gone, including all of `big_brain_derive`.
+* Measures and Weights are gone.
+* `big-brain` no longer exports everything from the toplevel. Use `big_brain::prelude::*` instead.
+
+#### Bug Fixes
+
+Probably.
+
+#### Features
+
+* New builder-based Thinker API!
+* Composite Scorers: `FixedScore`, `AllOrNothing`, and `SumOfScorers`.
+* Composite Action: `Steps`, for sequential Action execution.
+
+