Add changelog for version 0.3.0

CHANGELOG.md

+# Changelog for `micro-timer`
+
+## 0.3.0
+
+### User-facing
+* A compilation error is now generated if `#[timed]` is used on an async function
+* `#[timed]` now does not print anything if the function panics
+
+### Internal
+* Don't use `str::as_mut_ptr` in tests as it does not compile on `1.34.2`
+
+## Previous versions
+
+No changelog was kept for previous versions. Mostly it was behaviour fixing and experimentation, nothing of real value.