chore: update CHANGELOG.md for release

[scotthart]

This change is 

[codecov]

Codecov Report

Merging #1272 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1272      +/-   ##
==========================================
- Coverage   94.59%   94.57%   -0.02%     
==========================================
  Files         179      180       +1     
  Lines       13853    13863      +10     
==========================================
+ Hits        13104    13111       +7     
- Misses        749      752       +3

Impacted Files	Coverage Δ
...ud/spanner/integration_tests/client_stress_test.cc	81.57% <0%> (-1.76%)	⬇️
google/cloud/spanner/internal/spanner_stub.cc	64.28% <0%> (-1.03%)	⬇️
google/cloud/spanner/value.h	92.8% <0%> (-0.11%)	⬇️
google/cloud/spanner/internal/transaction_impl.h	100% <0%> (ø)	⬆️
google/cloud/spanner/internal/status_utils.cc	100% <0%> (ø)	⬆️
google/cloud/spanner/transaction.h	100% <0%> (ø)	⬆️
google/cloud/spanner/instance_admin_connection.h	100% <0%> (ø)

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