Document sub-resource Integrity (SRI) feature

[justinmayer]

Since the upstream webassets library added sub-resource Integrity (SRI) support via miracle2k/webassets#521, this plugin would benefit from some documentation regarding how to use that feature in a Pelican context.

(Original issue: getpelican/pelican#2875)

[justinmayer]

@egberts @jpralves: Would you be willing to help out by documenting webassets' SRI capabilities here in the Web Assets plugin?

[egberts]

I do not use JavaScript for security and privacy reasons, so this is not my wheelhouse nor my forte.