Update Readme with a compatibility note.

Readme.md

@@ -8,7 +8,44 @@
 
 This is go-generics,
 a collection of typesafe generic utilities
-for slices, maps, sets, iterators, and goroutine patterns in Go.
+for slices, maps, sets, and goroutine patterns in Go.
+
+# Compatibility note
+
+This is version 4 of this library,
+for the release of Go 1.23.
+
+Earlier versions of this library included a package,
+`iter`,
+that defined an iterator type over several types of containers,
+and functions for operating with iterators.
+However, Go 1.23 defines its own, better iterator mechanism
+via the new “range over function” language feature,
+plus [a new standard-library package](https://pkg.go.dev/iter) also called `iter`.
+This version of the go-generics library therefore does away with its `iter` package.
+The handy functions that `iter` contained for working with iterators
+(`Filter`, `Map`, `FirstN`, and many more)
+can now be found in the [github.com/bobg/seqs](https://pkg.go.dev/github.com/bobg/seqs) library,
+adapted for Go 1.23 iterators.
+
+(This version of go-generics might have kept `iter` as a drop-in replacement for the standard-library package,
+but was unable because the standard library defines two types,
+`iter.Seq[K]` and `iter.Seq2[K, V]`,
+that go-generics would have had to reference using type aliases;
+but Go type aliases [do not yet permit type parameters](https://github.com/golang/go/issues/46477#issuecomment-2101270785).)
+
+Earlier versions of this library included combinatorial operations in the `slices` package.
+Those have now been moved to their own library,
+[github.com/bobg/combo](https://pkg.go.dev/github.com/bobg/combo).
+
+Earlier versions of this library included two functions,
+`maps.Keys` and `maps.Values`,
+for producing a slice of (respectively) the keys of a map and the values of a map.
+As of Go 1.23,
+the standard library now defines its own `maps.Keys` and `maps.Values`,
+producing iterators instead of slices.
+This library updates those functions to match
+in order for its `maps` to remain a drop-in replacement for the stdlib `maps`.
 
 # Slices
 
@@ -20,9 +57,6 @@ The `slices` package is useful in three ways:
 - It includes `Map`, `Filter`, and a few other such functions
   for processing slice elements with callbacks.
 
-It also includes combinatorial operations:
-`Permutations`, `Combinations`, and `CombinationsWithReplacement`.
-
 The `slices` package is a drop-in replacement
 for the `slices` package
 added to the Go stdlib