Changelog: mention the shift to using cfg features

Author: aldanor

CHANGELOG.md

@@ -16,6 +16,12 @@
 
 - Renamed `filters::gzip_available()` to `deflate_available()` (the old name is
   present but marked as deprecated).
+- Code dependent on HDF5 version in `hdf5` and `hdf5-sys` crates now uses features
+  instead of cfg options: `cfg(feature = "1.10.1")` instead of `cfg(hdf5_1_10_1)`.
+  The main initial reason for that is for HDF5 versions to show up in the official
+  documentation on docs.rs.
+- Similar to the above, there's `have-direct`, `have-parallel` and `have-threadsafe`
+  features that reflect the build configuration of the underlying HDF5 library.
 
 ### Fixed