MINOR: Add documentation for running integration tests

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

I wanted to run the integration tests locally and it took a little figuring out so I am documenting it so that it is easier for others to use in the future.

What changes are included in this PR?

• Documentation
• Made Postgres env vars optional for more flexibility with different environments

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #2839 (30be839) into master (93d985c) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2839      +/-   ##
==========================================
- Coverage   85.20%   85.18%   -0.03%     
==========================================
  Files         274      275       +1     
  Lines       48543    48761     +218     
==========================================
+ Hits        41363    41539     +176     
- Misses       7180     7222      +42     

Impacted Files	Coverage Δ
datafusion/data-access/src/object_store/mod.rs	0.00% <0.00%> (-100.00%)	⬇️
datafusion/data-access/src/object_store/local.rs	78.06% <0.00%> (-10.97%)	⬇️
datafusion/core/src/physical_plan/stream.rs	66.66% <0.00%> (-9.53%)	⬇️
datafusion/core/src/datasource/file_format/avro.rs	61.53% <0.00%> (-8.03%)	⬇️
datafusion/core/src/datasource/file_format/json.rs	93.75% <0.00%> (-5.13%)	⬇️
.../core/src/physical_plan/file_format/file_stream.rs	89.89% <0.00%> (-3.35%)	⬇️
datafusion/core/src/physical_plan/join_utils.rs	93.61% <0.00%> (-3.20%)	⬇️
datafusion/common/src/error.rs	80.00% <0.00%> (-2.28%)	⬇️
...afusion/core/src/physical_plan/file_format/json.rs	91.06% <0.00%> (-2.13%)	⬇️
...tafusion/core/src/physical_plan/file_format/csv.rs	92.22% <0.00%> (-1.56%)	⬇️
... and 50 more

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[alamb]

Thank you @andygrove

cc @jimexist