docs(CHANGELOG): prepare for 2.0.0

CHANGELOG.markdown

@@ -1,3 +1,30 @@
+# [2.0.0](https://github.com/RedPRL/algaeff/compare/1.1.0...2.0.0) (2023-10-31)
+
+This major release has two breaking changes:
+
+1. `Algaeff.{Reader,Sequencer,State,UniqueID}` are now taking a module with a type `t`. Previously, the type is named `elt`, `env`, or `state` depending on the component. Now, it is always named `t`. The benefit is that one can write succinct code for built-in types:
+   ```ocaml
+   module R = Algaeff.Reader.Make (Bool)
+   module Sq = Algaeff.Sequencer.Make (Int)
+   module St = Algaeff.State.Make (Int)
+   module St = Algaeff.UniqueID.Make (String)
+   ```
+   To upgrade from the older version of this library, please change the type name (`env`, `elt`, or `state`) in
+   ```ocaml
+   module R = Algaeff.Reader.Make (struct type env = ... end)
+   module Sq = Algaeff.Sequencer.Make (struct type elt = ... end)
+   module St = Algaeff.State.Make (struct type state = ... end)
+   module U = Algaeff.UniqueID.Make (struct type elt = ... end)
+   ```
+   to `t` as follows:
+   ```ocaml
+   module R = Algaeff.Reader.Make (struct type t = ... end)
+   module Sq = Algaeff.Sequencer.Make (struct type t = ... end)
+   module St = Algaeff.State.Make (struct type t = ... end)
+   module U = Algaeff.UniqueID.Make (struct type t = ... end)
+   ```
+2. `Algaeff.Unmonad` is removed.
+
 # [1.1.0](https://github.com/RedPRL/algaeff/compare/1.0.0...1.1.0) (2023-10-01)
 
 ### Features