doc: update the INSTALL instructions

[coryan]

This change is 

[codecov]

Codecov Report

Merging #1093 into master will increase coverage by 0.44%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1093      +/-   ##
==========================================
+ Coverage   91.69%   92.13%   +0.44%     
==========================================
  Files         165      164       -1     
  Lines       12191    12187       -4     
==========================================
+ Hits        11178    11229      +51     
+ Misses       1013      958      -55

Impacted Files	Coverage Δ
google/cloud/spanner/internal/retry_loop.h	95.45% <0%> (-4.55%)	⬇️
google/cloud/spanner/internal/polling_loop.h	91.89% <0%> (-2.71%)	⬇️
google/cloud/spanner/value.h	92.7% <0%> (-0.11%)	⬇️
...anner/integration_tests/client_integration_test.cc	99.45% <0%> (-0.01%)	⬇️
google/cloud/spanner/database_admin_connection.h
google/cloud/spanner/client.cc	83.65% <0%> (+0.96%)	⬆️
...loud/spanner/internal/partial_result_set_resume.cc	95.45% <0%> (+4.54%)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	80.79% <0%> (+10.89%)	⬆️

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