add change log entry for bitvector package and remove stdpp_unstable/.keep

CHANGELOG.md

@@ -32,6 +32,11 @@ Coq 8.19 is newly supported by this version of std++.
   `{[ (x, y) : nat * nat | x = y ]}`. (by Thibaut Pérami)
 - Add `inv select` and `inversion select` tactics that allow selecting the
   to-be-inverted hypothesis with a pattern.
+- Create a `coq-stdpp-bitvector` package containing the previously
+  unstable `bitvector` library. Users of the library need to change
+  the import path from `stdpp.unstable.bitvector` to
+  `stdpp.bitvector.bitvector` and from `stdpp.unstable.bitvector_tactics` to
+  `stdpp.bitvector.tactics`.
 
 The following `sed` script should perform most of the renaming
 (on macOS, replace `sed` by `gsed`, installed via e.g. `brew install gnu-sed`).