chore: add SECURITY.md

[google-cloud-policy-bot]

add a security policy

[codecov]

Codecov Report

Merging #1105 (1b96728) into master (ce095f7) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1105      +/-   ##
============================================
- Coverage     84.91%   84.90%   -0.01%     
+ Complexity     2725     2722       -3     
============================================
  Files           156      156              
  Lines         14410    14410              
  Branches       1379     1379              
============================================
- Hits          12236    12235       -1     
  Misses         1607     1607              
- Partials        567      568       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../google/cloud/spanner/AbstractLazyInitializer.java	92.85% <0.00%> (-7.15%)	4.00% <0.00%> (-1.00%)
...ud/spanner/SessionPoolAsyncTransactionManager.java	85.03% <0.00%> (-1.58%)	13.00% <0.00%> (-2.00%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.01% <0.00%> (+0.19%)	74.00% <0.00%> (ø%)

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