METRON-1278 Strip "Build Status" widget from root README.md in site-book build

[mattf-apache]

Contributor Comments

See https://issues.apache.org/jira/browse/METRON-1278

Pull Request Checklist

For all changes:

• Is there a JIRA ticket associated with this PR? If not one needs to be created at Metron Jira.
• Does your PR title start with METRON-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.
• Has your PR been rebased against the latest commit within the target branch (typically master)?

For code changes:

N/A

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered by building and verifying the site-book?

[cestella]

+1 by inspection

[justinleet]

+1 by inspection. Thanks for grabbing this so quickly!

[JonZeolla]

+1 by inspection, thanks @mattf-horton

[mattf-apache]

Thanks, everyone. Committing...