Make it easier to find developer sign-off instructions

[michaelkaye]

Type of change

• Feature
• Bugfix
• Technical
• Other : update dangerfile

Content

Add link to synapse docs on how to provide DCO (which are linked elsewhere in the project) to the dangerfile message.

Motivation and context

Make it easier for third parties to contribute by making it more obvious how to sign off (and that it's not a lengthy Process).

Screenshots / GIFs

Tests

This PR did not have a DCO signoff.

Checklist

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

Signed-off-by: Michael Kaye 1917473+michaelkaye@users.noreply.github.com

[michaelkaye]

The danger failure is an old failure that has been corrected.
the lint failure is an OOM that is being addressed elsewhere - think we're good to force merge this?

[ouchadam]

@michaelkaye yep! the lint issue should be fixed with #6728 (just merged)

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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