Update draft-hegde-ospf-group-admin-tag.xml

1.Added section 5 in applications; text provided by bruno
2.updated section 4.2 with below statements
"Node administrative tags 
	are for generic use and do not require IANA registry. Any new feature
	requiring well known tag values MAY use capability TLV as defined in
	<xref target="RFC4970"/>."

3. Addresses review comments raised in WG adoption call