Add .gitignore, links to GeoJSON.io and GeoJSONLint

There are three parts:

- The .DS_Store file is going to show up in all directories on Mac. Fortunately you can add a .gitignore file, which will tell git to avoid files like .DS_Store, files with passwords, etc. This commit removes .DS_Store and adds a .gitignore

- Adds a GeoJSON.io link.

- Because of how README markdown works on GitHub, (5a) and (5b) got absorbed into (4), and the ones that you've labeled 6-20 then got re-labeled as 5-19. I put (5a) and (5b) into (5) so the steps in the README markdown and on GitHub match.

- Added link to GeoJSONLint, a site to verify that you're writing valid GeoJSON.