chore: add SECURITY.md

[google-cloud-policy-bot]

add a security policy

[codecov]

Codecov Report

Merging #720 (8fa9b57) into master (a705654) will increase coverage by 0.15%.
The diff coverage is n/a.

❗ Current head 8fa9b57 differs from pull request most recent head 4b44e38. Consider uploading reports for the commit 4b44e38 to get more accurate results

@@            Coverage Diff             @@
##           master     #720      +/-   ##
==========================================
+ Coverage   91.27%   91.43%   +0.15%     
==========================================
  Files         140      133       -7     
  Lines       14952    13874    -1078     
  Branches     1074     1031      -43     
==========================================
- Hits        13648    12686     -962     
+ Misses       1011      907     -104     
+ Partials      293      281      -12     

Impacted Files	Coverage Δ
...n/AbstractServiceCallableFactoryClassComposer.java	83.57% <0.00%> (-16.43%)	⬇️
...va/com/google/api/generator/gapic/model/Field.java	54.83% <0.00%> (-7.67%)	⬇️
...google/api/generator/gapic/protoparser/Parser.java	44.60% <0.00%> (-1.83%)	⬇️
...i/generator/engine/writer/ImportWriterVisitor.java	91.91% <0.00%> (-0.86%)	⬇️
.../com/google/api/generator/engine/ast/TypeNode.java	91.60% <0.00%> (-0.77%)	⬇️
...poser/common/AbstractServiceStubClassComposer.java	97.79% <0.00%> (-0.57%)	⬇️
...apic/composer/comment/SettingsCommentComposer.java	93.93% <0.00%> (-0.18%)	⬇️
...ic/composer/common/ServiceClientClassComposer.java	98.45% <0.00%> (-0.18%)	⬇️
.../composer/grpc/ServiceClientTestClassComposer.java	93.56% <0.00%> (-0.04%)	⬇️
.../resourcename/ResourceNameHelperClassComposer.java	91.96% <0.00%> (-0.03%)	⬇️
... and 20 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a705654...4b44e38. Read the comment docs.