Update changelog

Author: awvwgk

CHANGELOG.md

@@ -10,6 +10,9 @@ Features available from the latest git source
 - new module `stdlib_version`
   [#579](https://github.com/fortran-lang/stdlib/pull/579)
   - new procedure `get_stdlib_version`
+- update module `stdlib_io`
+  [597](https://github.com/fortran-lang/stdlib/pull/597)
+  - new procedure `getline`
 - new module `stdlib_io_npy`
   [#581](https://github.com/fortran-lang/stdlib/pull/581)
   - new procedures `save_npy`, `load_npy`