[BigQueryWorkloadTester] Updating BigQuery Workload Tester's README with additional info about concurrency limits and results.

[codecov[bot]]

Codecov Report

Merging #209 into dev will increase coverage by 5.25%. The diff coverage is n/a.

@@             Coverage Diff             @@
##               dev     #209      +/-   ##
===========================================
+ Coverage     78.4%   83.65%   +5.25%     
+ Complexity     542      198     -344     
===========================================
  Files           68       34      -34     
  Lines         4339     1083    -3256     
  Branches       330       64     -266     
===========================================
- Hits          3402      906    -2496     
+ Misses         837      168     -669     
+ Partials       100        9      -91

Impacted Files	Coverage Δ	Complexity Δ
.../cloud/pontem/BaseCloudSpannerDatabaseRestore.java
.../cloud/pontem/AvroCloudSpannerDatabaseRestore.java
...loud/pontem/FormatStructAsGenericRecordFnTest.java
...oud/pontem/AvroCloudSpannerDatabaseBackupTest.java
...ntem/CloudSpannerDatabaseBackupIntegrityCheck.java
...tem/FormatSpannerTablesListStructAsTextFnTest.java
...java/com/google/cloud/pontem/TableInformation.java
...CloudSpannerDatabaseRestoreIntegrityCheckTest.java
...rc/test/java/com/google/cloud/pontem/UtilTest.java
...e/cloud/pontem/BaseCloudSpannerDatabaseBackup.java
... and 24 more