Update README.md with limitations (#229)

- Fixes #194

README.md

@@ -54,3 +54,9 @@ After installing, go to `Advanced Settings` -> `Physics` -> `2D` or `3D`. Change
 # Implementation Progress
 
 This plugin is still being developed. See the [Implementation Progress](https://godot.rapier.rs/docs/progress/) to get an idea of what status it is in and what features it has.
+
+# Limitations
+
+- Pin Softness is not supported
+- Double builds are disabled for now (until salva supports double builds)
+- No support for asymetric collisions (eg. object 1 hitting object 2 but object 2 not hitting object 1). This is the exact check rapier does: `(A.layer & B.mask) != 0 || (B.layer & A.mask) != 0`