chore: add SECURITY.md

[google-cloud-policy-bot]

add a security policy

[codecov]

Codecov Report

Merging #292 (e7f7054) into master (efdc875) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master    #292      +/-   ##
===========================================
- Coverage      8.24%   8.19%   -0.05%     
  Complexity      410     410              
===========================================
  Files            85      85              
  Lines         28545   28545              
  Branches       4920    4920              
===========================================
- Hits           2353    2340      -13     
- Misses        25904   25907       +3     
- Partials        288     298      +10     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/grafeas/v1/GrafeasClient.java	78.87% <0.00%> (-4.32%)	82.00% <0.00%> (ø%)
...n/java/io/grafeas/v1/stub/GrafeasStubSettings.java	81.67% <0.00%> (-1.20%)	24.00% <0.00%> (ø%)

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