add CONTRIBUTING info for security issues

[riyazdf]

Mimicking other docker projects.

cc @stevvooe @dmcgowan @aaronlehmann

Signed-off-by: Riyaz Faizullabhoy [email protected]

[codecov]

Codecov Report

Merging #2346 into master will decrease coverage by 9.61%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2346      +/-   ##
==========================================
- Coverage   60.23%   50.62%   -9.62%     
==========================================
  Files         126      126              
  Lines       14390    14390              
==========================================
- Hits         8668     7285    -1383     
- Misses       4834     6357    +1523     
+ Partials      888      748     -140

Impacted Files	Coverage Δ
registry/storage/driver/gcs/gcs.go	0.32% <0%> (-66.13%)	⬇️
registry/storage/driver/oss/oss.go	0.45% <0%> (-56.5%)	⬇️
registry/storage/driver/s3-aws/s3.go	4.07% <0%> (-55.4%)	⬇️
registry/storage/driver/s3-goamz/s3.go	0.4% <0%> (-52.4%)	⬇️
registry/client/transport/transport.go	68.75% <0%> (-8.75%)	⬇️

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[stevvooe]

LGTM

[aaronlehmann]

LGTM