Document the write diagnostics and VR mismatch in the README.

[xbmlz]

v0.28.0 shipped two diagnostics the README never mentioned.

WriteOptions did not appear in the README at all, so the write-side hook — the
headline of #67 — was discoverable only from the changelog or godoc. The
read-side section still listed the 0.27.0 set of anomalies, with no VR
disagreement (#68), and said a diagnostic carries its "enclosing sequences",
which stopped being the whole truth when Diagnostic.Path started naming which
item of each (#69). Adding the README paragraph in the same commit as the
feature is what #54 did for read diagnostics; I missed it three times in a row.

The example changed shape because the obvious one is impossible

The changelog leads the write diagnostics with a fractional float64 in an
IS, so that is what I first wrote the README example on. It cannot happen
through the typed API:

SetFloat: godicom: tag (0018,0086) has VR IS, which does not hold
floating-point values

SetFloat rejects it at the call site, so an example written that way would
never reach the hook it was demonstrating. I checked each construction against
the real API before picking one:

construction	result
SetInt(EchoNumbers, 3000000000)	reaches the writer, reports is outside [-2147483648, 2147483647]
SetString(SliceThickness, "0.3333333333333333")	reaches the writer, reports is 18 bytes, over the 16 a DS allows
Set(NewDataElement(EchoNumbers, VRIS, 1.5))	reaches the writer, reports "1.5" is not an integer string
SetFloat(EchoNumbers, 1.5)	rejected at the call site
SetFloat(SliceThickness, 1.0/3.0)	reports nothing — 0.28.0 truncates it per FormatNumberAsDS

So the example uses SetInt, the comment under it is the diagnostic's real
output byte for byte, and the section closes on the boundary that last row
implies: the dictionary-VR setters reject what they can see themselves, and the
hook covers what they cannot.

No behaviour change — README.md and a Docs: changelog entry under
Unreleased, following the v0.26.0 and v0.24.0 precedent for doc-only entries.
The three reported cases above are already pinned by write_diagnostic_test.go,
so I did not add a duplicate test; the repo has no Example functions or
README-snippet harness, and introducing one is a bigger call than this fix.