Reorganizing README to be compliant with DockerHub upload limit

[brycahta]

Issue #, if available: N/A

Description of changes:

• Updated README.md to be within DockerHub upload character count limit (25000). Updated char count: 20911
• Added readme-validator to ensure readme files are below max char count-- this will be invoked by travisci for all PRs
• Updated sync-readme-to-dockerhub to use $DOCKERHUB_PASSWORD instead of TOKEN as the token is not supported per this issue
• Kept common use cases and high-level descriptions in main README and moved advanced usage and configuration details to docs/usage.md and docs/configuration.md, respectively.
• When adding more installation methods, we can create installation.md at that time if we are approaching char limit in main README.
• Although the main README.md will be the only one synced with DockerHub, validating all .md files to keep them from getting too lengthy

Testing:

• make clean && make build && make test succeed
• tested make sync-readme-to-dockerhub on new README with private repo; upload was successful
• tested make validate-readme on current ReadMes and passed. Also tested on old README (exceeding char count) and test failed as expected

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[pdk27]

Nice refactor. LGTM, left some minor comments.

[pdk27]

Looks good 👍