update readme

[dwhieb]

Update the readme to focus more specifically on ALTLab's fork.

• links to Spraakbanken's documentation
• ALTLab's branching structure
• building and testing the app locally
• fix status badge (and add new ones)
• ALTLab-specific configuration settings (relationship between config.js, common.js, and default_mode.js); also add inline documentation for this

Our dev branch should actually update from Spraakbanken's master branch, because that's more well tested. We should avoid pulling from Spraakbanken's dev branch if possible. (See spraakbanken#160.) We'll keep a running dev branch, which issue branches should be made from.