[httpslocal/usecases] Update RelevantSpecs.md (#25)

This PR adds several references to IETF documents listed in RelevantIETFDocuments.

@dajiaji @yoneyajp @igarashi50 Could you review this and recommend additional references if any?
You can view, comment on, or merge this pull request online at:

  https://github.com/httpslocal/usecases/pull/25

-- Commit Summary --

  * update RelevantSpecs.md

-- File Changes --

    M README.md (2)
    M RelevantSpecs.md (28)

-- Patch Links --

https://github.com/httpslocal/usecases/pull/25.patch
https://github.com/httpslocal/usecases/pull/25.diff

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