feat: announce move to monorepo

[devjgm]

Part of googleapis/google-cloud-cpp#3591

Preview: https://github.com/devjgm/google-cloud-cpp-spanner/tree/readme-announce

@scotthart I think you'll want to include a blurb like this one in the April release.

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This change is 

[codecov]

Codecov Report

Merging #1455 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1455      +/-   ##
==========================================
- Coverage   95.54%   95.53%   -0.01%     
==========================================
  Files         190      190              
  Lines       15782    15796      +14     
==========================================
+ Hits        15079    15091      +12     
- Misses        703      705       +2

Impacted Files	Coverage Δ
google/cloud/spanner/internal/log_wrapper.h	75% <0%> (-9.62%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	85.71% <0%> (-2.07%)	⬇️
...nner/benchmarks/single_row_throughput_benchmark.cc	90.78% <0%> (-0.03%)	⬇️
google/cloud/spanner/internal/session.h	100% <0%> (ø)	⬆️
google/cloud/spanner/internal/transaction_impl.h	100% <0%> (ø)	⬆️
...cloud/spanner/internal/partial_result_set_resume.h	100% <0%> (ø)	⬆️
google/cloud/spanner/session_pool_options.h	100% <0%> (ø)	⬆️
google/cloud/spanner/transaction.h	100% <0%> (ø)	⬆️
...oogle/cloud/spanner/internal/instance_admin_stub.h
google/cloud/spanner/instance_admin_connection.h	100% <0%> (ø)
... and 3 more

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