update README

README.md

@@ -9,6 +9,7 @@ file on the search path (note that this include Lean 4 and all dependencies of y
 
 You can also use `lake exe lean4checker --fresh Mathlib.Data.Nat.Basic` to replay all the constants
 (both imported and defined in that file) into a fresh environment.
+This is single threaded, and may be much slower.
 
 This is not an external verifier, as it uses the Lean kernel itself.
 However it is useful as a tool to detect "environment hacking",
@@ -33,10 +34,3 @@ Despite `.olean` files being "fairly cross-platform",
 `lean4checker` will reject `.olean`s that were compiled on a system
 that  does not use the same bignum library as your system,
 so it advisable to not rely on cached `.olean`s.
-
-## Variants
-
-It would be easy to adapt this code to replay the entire environment up to some set of declarations,
-starting from an empty environment
-(rather than the current implementation, which starts from the environment provided by the imports).
-If this would be useful to you, please ask.