Prevent schema from accepting null values for keyword

keyword should always be required. This was an oversight in the update
on #269
Improve JSON schema for distribution, keywords, PrimaryITInvestmentUII
Adds accessURL and format as possible fields outside of distribution
Fixed some rules to prevent empty arrays/strings
Added a json to csv and a csv to json tool
Relaxing requirement for keyword uniqueness as per #288
Update policy-docs.md
Makes site protocol-neutral (possible to run over HTTP or HTTPS)
accessURL and format can be null outside of a distribution
Update CONTRIBUTING.md

No where on the github repo could I find a link to the site. I had to look through the pull requests to find out how to see the content. I added a link on the contribute page that provides at least a little information on how to get to the site.
Merge pull request #290 from project-open-data/adding-new-tools

Added a json to csv and a csv to json tool
Merge pull request #294 from project-open-data/add-gsa-data-policy

Adds a GSA open data document
Merge pull request #286 from philipashlock/json-schema-requirements-fix

JSON schema corrections
Merge pull request #295 from konklone/https_ready

Makes site protocol-neutral (possible to run over HTTP or HTTPS)
Merge pull request #298 from jsturdevant/patch-1

Added URL to site on CONTRIBUTING.md
adding definition of data asset
adding a usage note to keywords
Merge pull request #300 from project-open-data/clarify-guidance-around-duplicate-keywords

adding a usage note to keywords
Merge pull request #299 from project-open-data/adding-data-asset-definition-to-glossary

adding definition of data asset
change links for two tools

For the json validator and catalog generator.