Update README.md

- Adds Readme structure inspired by CFPB project template at
https://github.com/cfpb/open-source-project-template.
- Adds more comprehensive installation instructions (of note, there
were no instructions for starting Elasticsearch).
- Removes copy that talks about how Wordpress was used in the project
in the past.
- Removes copy that references GitHub Enterprise.
- Cleans up unneeded tab characters.
- Moves the working with templates section under the Getting Involved
section.

Additionally fixes:
#247 (comment),
#247 (comment),
#247 (comment),
#247 (comment),
#247 (comment),
#247 (comment),
#247 (comment),
#247 (comment)

- Adds missing `$` in terminal code snippets.

More additional fixes:
#247 (comment),
#247 (comment),
#247 (comment),
#247 (comment)