Merge branch 'feat/p3-in-place-modify' into feat/p3-range-zfp-edit

# Conflicts:
#	CHANGELOG.md
#	crates/clawhdf5-py/src/lib.rs
#	crates/clawhdf5/src/error.rs

README.md

@@ -433,6 +433,23 @@ b.write("groups.h5")?;
 A group holds at most 65 535 links; more is an error, as is a link over
 65 515 bytes (a very long soft-link target) in a group of more than 8 links.
 
+### Modifying an existing file
+
+```rust
+use clawhdf5::{AttrValue, FileEditor, Selection};
+
+// A file from h5py or clawhdf5, dataset "x" chunked with maxshape=(None,).
+let mut ed = FileEditor::open("data.h5")?; // exclusive lock, like libhdf5
+ed.resize("x", &[1100])?;                   // h5py: ds.resize((1100,))
+let sel = Selection::Hyperslab { start: vec![1000], stride: vec![1], count: vec![100], block: vec![1] };
+ed.write_values("x", &sel, &[0.5f64; 100])?; // ds[1000:1100] = 0.5
+ed.set_attr("x", "units", &AttrValue::String("m/s".into()))?;
+```
+
+Each call changes the file in place (no rewrite) and syncs it. What it
+cannot change safely is refused before anything is written; see
+[known issues](docs/known-issues.md) for the limits.
+
 ### Python
 
 `crates/clawhdf5-py` is a Python package (PyO3 + numpy) that reads HDF5 with