collapse_vars: document option in README

README.md

@@ -323,6 +323,9 @@ to set `true`; it's effectively a shortcut for `foo=true`).
 - `cascade` -- small optimization for sequences, transform `x, x` into `x`
   and `x = something(), x` into `x = something()`
 
+- `collapse_vars` -- default `false`. Collapse single-use var and const
+  definitions when possible.
+
 - `warnings` -- display warnings when dropping unreachable code or unused
   declarations etc.