Update README.md to include additional information on expand-data-corpus.py beta feature (#92)

[IanHoang]

Description

This PR quickly adds an extra sentence explaining what directory expand-data-corpus.py influences.

Issues Resolved

#92

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[ayushav12]

Let's add some more details in the documentation about provision of defining private workloads and how it differs from "default" workload.

[IanHoang]

I'll also add info in the README.md there's already a parameter to specify repository to make the changes in.

Usage:

    Use the -h option to view script usage and options.

options:
  -h, --help            show this help message and exit
  -w WORKLOAD, --workload WORKLOAD
                        workload name, default: http_logs
  -r WORKLOAD_REPOSITORY, --workload-repository WORKLOAD_REPOSITORY
                        workload name, default: default
  -c CORPUS_SIZE, --corpus-size CORPUS_SIZE
                        size of corpus to generate in GB
  -o OUTPUT_FILE_SUFFIX, --output-file-suffix OUTPUT_FILE_SUFFIX
                        suffix for output file name, documents-SUFFIX.json, default: generated
  -f INPUT_FILE, --input-file INPUT_FILE
                        [EXPERT] input file name, default: documents-241998.json
  -n NUMBER_OF_DOCS, --number-of-docs NUMBER_OF_DOCS
                        [EXPERT] number of documents to generate
  -i INTERVAL, --interval INTERVAL
                        [EXPERT] interval between consecutive timestamps, use a negative number to specify multiple docs per timestamp
  -t START_TIMESTAMP, --start-timestamp START_TIMESTAMP
                        [EXPERT] start timestamp, default: 893964618
  -b BATCH_SIZE, --batch-size BATCH_SIZE
                        [EXPERT] batch size per OSB client thread, default: 50000

[gkamat]

nit: "the" http_logs workload

[IanHoang]

Let's add some more details in the documentation about provision of defining private workloads and how it differs from "default" workload.

@ayushav12 this is a good idea. However, we can add this to the official documentation as this README is pertains to the official workloads (default workloads) that come with OSB