Map specs in RelevantIETFDocuments onto RelevantSpecs.md

[tomoyukilabs]

This issue is a meta issue to assign mapping specs in RelevantIETFDocuments onto RelevantSpecs.md to myself.

[tomoyukilabs]

@yoneyajp has made a great contribution about IETF activities in #17, so it should be absolutely valuable to refer to and review these specs.