Adds `rel` parameter to the social icons configuration documentation (#987)

Alexander Lazarević

3 days ago eea6dfd3fe4b97518d2c5d2fb550e906793507e8

1
<a href="{{ .Destination | safeURL }}"{{ with .Title}} title="{{ . }}"{{ end }} {{ if findRE "^(https?:)?//" (lower .Destination) }} class="external-link" target="_blank" rel="noopener"{{ end }}>{{ .Text | safeHTML }}</a>