Document recommendation for generating bzlmod integrity value

Currently the documentation for the `integrity` field:

> integrity: The [Subresource Integrity](https://w3c.github.io/webappsec-subresource-integrity/#integrity-metadata-description) checksum of the archive.

links to the spec of the subresource integrity format. I think ideally given the community has generally standardized on using sha256s of the archives, there should be a quick snippet recommendation folks can copy and paste to generate what they need. I guess that is something like:

```
openssl dgst -sha256 -binary ARCHIVE.tar.gz | openssl base64 -A | sed 's/^/sha256-/'
```