Update vendor directory to match expectation of vndr tool

[dmcgowan]

Adds READMEs and enforces vendor is done at repository root

[codecov]

Codecov Report

Merging #2241 into master will decrease coverage by 10.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2241       +/-   ##
===========================================
- Coverage   61.64%   51.61%   -10.03%     
===========================================
  Files         125      125               
  Lines       11419    11417        -2     
===========================================
- Hits         7039     5893     -1146     
- Misses       3493     4777     +1284     
+ Partials      887      747      -140

Impacted Files	Coverage Δ
registry/storage/driver/gcs/gcs.go	0.4% <0%> (-68.94%)	⬇️
registry/storage/driver/oss/oss.go	0.56% <0%> (-57.23%)	⬇️
registry/storage/driver/s3-aws/s3.go	4.6% <0%> (-56.18%)	⬇️
registry/storage/driver/s3-goamz/s3.go	0.5% <0%> (-50.76%)	⬇️
registry/client/transport/transport.go	69.69% <0%> (-9.1%)	⬇️
configuration/configuration.go	68.33% <0%> (-0.27%)	⬇️

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[stevvooe]

LGTM