Rename readme.md files to README.md to match convention (#1516)

Follow-up from #1484

The `README.md` name is:

1) Standard practise, even for non-package READMEs. For example, see:
https://github.com/iced-rs/iced/blob/master/examples/README.md; and
https://github.com/bevyengine/bevy/blob/main/examples/README.md
2) Much more obvious in a file list (I didn't even know we had
`masonry/examples/readme.md`, as it blends in with the example files
when browsing in a code editor).
3) Matches existing practise within this same repo (e.g.
xilem/resources/fonts/roboto_flex, et al.) and in other Linebender repos
(incl. Vello).

This PR corrects the landing of a change which didn't get review
approval.

Author: DJMcNab