Document how to contribute to this repo

[gosko]

Now that /ns/ is maintained on GitHub, it would be good to document the process for contributors to follow.

There is a related issue within the Guidebook repo but some of the tech details might be better documented here, either in README.md or in a CONTRIBUTING file.

Things that would be worthwhile to include in those docs:

• additions/changes must be submitted as pull requests
• proxying requests elsewhere is not allowed
• Apache config details:
  • Options MultiViews is enabled
  • AllowOverrideList is: <Files <FilesMatch DirectoryIndex ForceType Header RewriteBase RewriteCond RewriteEngine RewriteRule
  • the list of CORS-safe MIME types for this area is:
    • application/json
    • application/ld+json
    • application/rdf+xml
    • text/csv
    • text/n3
    • text/shex
    • text/turtle
    • font/woff
    • font/woff2