Update README.md

[otobongfp]

Updated new page provided by adobe that talks about digital or e-signatures.

[dhensby]

The current link is 404-ing but I don't think the linked to page is an equivalent at all.

It may be better just to remove the link to the adobe doc completely and just stick with the ETSI standard...

[vbuch]

Already addressed some time ago and forgot to close here.