message: Document the supported extensions
diff: # Type Checker
  Bril comes with a simple type checker to catch errors statically.
  -It checks the types of instructions in the [core language](../lang/core.md) and some extensions, calls and return values, and the labels used in control flow.
  +It checks the types of instructions in the [core language](../lang/core.md) and the [floating point](../lang/float.md), [SSA](../lang/ssa.md), [memory](../lang/memory.md), and [speculation](../lang/spec.md) extensions.
  +It also checks calls and return values and the labels used in control flow.
  Install
  -------
  