Add changelog

Adds changelog entries for the new 1.3.0 release and also for the
previous 1.2.0 release. I did not bother documenting previous releases.

CHANGELOG.md

@@ -0,0 +1,19 @@
+## 1.3.0
+
+- Replace hash with faster and better finalized hash.
+  This replaces the previous "fxhash" algorithm originating in Firefox
+  with a custom hasher designed and implemented by Orson Peters ([`@orlp`](https://github.com/orlp)).
+  It was measured to have slightly better performance for rustc, has better theoretical properties
+  and also includes a signficantly better string hasher.
+- Fix `no_std` builds
+
+## 1.2.0 (**YANKED**)
+
+**Note: This version has been yanked due to issues with the `no_std` feature!**
+
+- Add a `FxBuildHasher` unit struct
+- Improve documentation
+- Add seed API for supplying custom seeds other than 0
+- Add `FxRandomState` based on `rand` (behind the `rand` feature) for random seeds
+- Make many functions `const fn`
+- Implement `Clone` for `FxHasher` struct