Add vc-data-integrity as a non-browser spec.

[jyasskin]

FYI @msporny, this should help with w3c/vc-di-ecdsa#57.

[msporny]

@jyasskin thank you!

Please help me understand what adding vc-data-integrity to browser-specs does? I read the repo README.md and couldn't really tell, tried seeing what pulls this repo in upstream (and couldn't find anything). I thought that maybe this helps ReSpec xref do it's job, but that doesn't seem right, either? IOW, I tried to figure out how this helps but kept hitting dead-ends.

If you tell me, I'm happy to do a PR against this repo's README.md to clarify why listing a spec in this repo is "A Good Thing To Do"(tm).

[dontcallmedom]

@msporny the specs catalogued in this repo are the ones that are crawled regularly by reffy to produce the extracts published in webref, including the definitions that are used by ReSpec and bikeshed automatic links.

(the repo is called browser-specs because it was historically the focus of that catalogue; it is now open to any spec that can reasonably be qualified as a web spec)