Update pubspec so that using this repo as a git dependency works

[Rexios80]

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• [x] The code changes follow best practices
• [ ] Code changes are tested (add some information if not applicable)

Description of the pull request

• Updates stream_feed_flutter/pubspec.yaml so that it can be used as a git dependency
• Updates stream_feed_flutter/README.md to reflect a more responsible use of git dependencies

I looked through the major dependency upgrades and it looks like nothing should have broken. The commit that was used for the photo_view dependency is included in the latest release.