docs(readme): Fix description of cargo feature flag

The feature flags are not mutually exclusive anymore.

README.md

@@ -90,11 +90,11 @@ heron = { version = "0.11.1", features = ["2d"] }
 
 ## Feature flags
 
-One must choose to use either `2d` or `3d` (but not both). If none of theses two features is enabled, the `PhysicsPlugin` won't be available.
+One must choose to use either `2d` or `3d`. If none of theses two features is enabled, the `PhysicsPlugin` won't be available.
 
 
-* `3d` Enable simulation on the 3 axes `x`, `y`, and `z`. Incompatible with the feature `2d`.
-* `2d` Enable simulation only on the first 2 axes `x` and `y`. Incompatible with the feature `3d`, therefore require to disable the default features.
+* `3d` Enable simulation on the 3 axes `x`, `y`, and `z`.
+* `2d` Enable simulation only on the first 2 axes `x` and `y`.
 * `debug-2d` Render 2d collision shapes. Works only in 2d, support for 3d may be added later.