DMARCbis WGLC: Add reference to Section 8.6 to end of section 7.6

[toddherr]

On-list discussion of "General Purpose Domain" leads to this proposal.

Section 7.6 ends with the following sentence:

In particular, this document makes explicit that domains for general-purpose email **SHOULD NOT** deploy a DMARC policy of p=reject.

The proposal is to tack on an additional sentence that reads:

See section 8.6 (Interoperability Considerations) for further discussion of this topic.

or words to that effect.

[toddherr]

update made and pushed to working branch.