docs: Update FEATURES.md

[emmambd]

Summary:

Add associated gtfs.org documentation link to the features markdown.

[welcome]

Thanks for opening this pull request! You're awesome. We use semantic commit messages to streamline the release process. Before your pull request can be merged, you should update your pull request title to start with a semantic prefix. Examples of titles with semantic prefixes:

• fix: Bug with ssl network connections + Java module permissions.
• feat: Initial support for multiple @PrimaryKey annotations.
• docs: update RELEASE.md with new process
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• Include tests when adding/changing behavior
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