docs(readme): note the 5.10.3 upgrade behaviour change

[Ravenwater]

The v5.10.3 release page now carries the full explanation of the .noconvert() change — but PyPI renders the README, not release notes. Someone running pip install -U mtl5 had no way to find out that vector()/matrix() can now raise on input that worked in 5.10.2.

Adds a short ### Upgrading to 5.10.3 under Install, where someone who just upgraded will actually look. It covers three things and links out for the rest:

• who is affected — stated as concrete inputs (a non-contiguous array: a[::2], a column, a transpose; or a dtype outside the registered nine) rather than as a description of the change
• the one-line fix — np.ascontiguousarray(a), framed as preserving precision rather than as a workaround, since that is what it does
• why it is a fix, not a restriction — the old path silently downcast a float64 slice to float32 while reporting is_view=True and aliasing nothing

Docs only. Lands as docs:, which is not in patch_tags, so it cuts no release.

Worth dropping this section once 5.10.3 is a few releases back — noted in the commit message so whoever prunes the README knows it was meant to be temporary.

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