Update with recent motions (image-spec v0.5.0, etc.)

Update with recent motions (image-spec v0.5.0, etc.)

Although the image-spec vote drifted significantly between the initial
proposal to tag 67521f5 (based on 72d0525) and the tagged aed55bc.
Changes in that gap that were ambiguously covered by the vote:

  image-spec$ git log --graph --oneline --decorate 67521f5..aed55bc
  * aed55bc (tag: v0.5.0) version: v0.5.0
  *   8d0ee71 Merge pull request #257 from coolljt0725/fix_media_type
  |\
  | * 0a76d61 (origin/pr/257) Add license head to manifest_test.go
  | * 5c9e424 Fix media type on unpacking layer
  *   61989fe Merge pull request #278 from xiekeyang/error-value
  |\
  | * 85bbf42 (origin/pr/278) improper error description
  *   8c2642e Merge pull request #290 from stevvooe/better-wording-to-introduce-media-types
  |\
  | * d5fd2d8 (origin/pr/290) media-type: clarify that we have MIME-compatible media types
  * |   ec62652 Merge pull request #291 from stevvooe/break-sentence
  |\ \
  | * | c2b9e7f (origin/pr/291) media-types: break up sentence into multiple lines
  | |/
  * |   d564f8e Merge pull request #284 from TrumanLing/fix-typo
  |\ \
  | * | 6304f58 (origin/pr/284) README: Fix "can then discovered" -> "can then be discovered"
  |  /
  * |   5553ed5 Merge pull request #221 from x1022as/descriptor
  |\ \
  | |/
  |/|
  | * 75a51ed (origin/pr/221) fix regular expression of algorithim
  * 5a0447a Merge pull request #169 from runcom/layer-urls
  * 64e7a30 (origin/pr/169) descriptor: add urls property

The runtime-spec vote (nominally for c678086) was closer to the
target, tagging 7dab1a2:

  runtime-spec$ git log --graph --oneline --decorate c678086^..7dab1a2
  * 7dab1a2 (tag: v1.0.0-rc2) version: 1.0.0-rc2
  * 009e969 (origin/release-v1.0.0-rc2) ChangeLog: Document changes since v1.0.0-rc1
  * c678086 Merge pull request #553 from RobDolinMS/patch-11
  * 9da8967 (origin/pr/553) [ReadMe] Just one level/tier of compliance (rebase)