Add instructions for filtering by labels and repo using URL

[digitaldina]

Fixes #72

• [x ] tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with rake test
• code is in uniquely-named feature branch and has no merge conflicts
• [ x] PR is descriptively titled
• [ x] PR body includes fixes #0000-style reference to original issue #
• [ x] ask @publiclab/reviewers for help, in a comment below

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[digitaldina]

@publiclab @Rishabh570 please review!

[jywarren]

Looks fantastic, thank you!

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