Use Subresource Integrity in installation instructions

[e2jk]

This adds the integrity and crossorigin attributes to the CDN install instructions. BootstrapCDN implented this in 2015 already.

Copied from https://www.srihash.org/ (which I used to calculate these hashes): SRI is a new W3C specification that allows web developers to ensure that resources hosted on third-party servers have not been tampered with. Use of SRI is recommended as a best-practice, whenever libraries are loaded from a third-party source.

Learn more about how to use subresource integrity on MDN.