Fix Pillow error, add helpful .gitignore, conform models.md to markdownlint when using GAE deploy starter

[SwiftWinds]

• There is a breaking change in Pillow 7.0.0, so 6.2.2 has to be installed instead (Pillow replaced PILLOW_VERSION with __version__)

• A .gitignore can make it less likely that individuals accidentally add .pth, .DS_STORE, etc. files to their git repos

• The first line in models.md was changed to a top-level heading to conform with markdown best practices

The repo of the changes can be found here: https://github.com/SwiftWinds/google-app-engine