mention the features in the README

Author: nikomatsakis

Cargo.toml

@@ -4,7 +4,7 @@ description = "Union-find, congruence closure, and other unification code. Based
 license = "MIT/Apache-2.0"
 homepage = "https://github.com/nikomatsakis/ena"
 repository = "https://github.com/nikomatsakis/ena"
-version = "0.5.0"
+version = "0.6.0"
 authors = ["Niko Matsakis <niko@alum.mit.edu>"]
 
 [features]

README.md

@@ -10,6 +10,16 @@ code.
 
 The name "ena" comes from the Greek word for "one".
 
+### Features
+
+By default, you just get the union-find implementation. You can
+also opt-in to the following experimental features:
+
+- `bench`: use to run benchmarks (`cargo bench --features bench`)
+- `congruence-closure`: opt-in to the congruence closure
+  implementation. This requires petgraph and is nightly only (since
+  it uses `impl Trait`).
+
 ### License
 
 Like rustc itself, this code is dual-licensed under the MIT and Apache