Remove obsolete note

[kgryte]

This PR

• removes an obsolete note concerning adoption. The note referenced NEP 37; however, that proposal has not seen the requisite movement necessary to change either the specification's use of __array_namespace__ or the experimental implementation in NumPy. Accordingly, this PR chooses to remove the note to avoid creating any uncertainty regarding the preferred mechanism of exposing a conforming array implementation as a separate namespace.

cc @rgommers for visibility.

[kgryte]

Will merge. If further changes are necessary, they can be made in follow-up PRs.

[rgommers]

Agreed with removal. NEP 37 was useful inspiration, but it never went anywhere so there's no point in referencing it.