Create README.md

Author: marwaneltoukhy

README.md

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+# Central CI
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+This CI tests the user flow for several designs at once, it is made to have the latest most reliable set of tool versions to use in the user flow.
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+Until now there are 2 designs being run: `caravel_user_project` and `openframe_timer_example`
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+There are 2 CIs, one that runs nightly, and updates the tool versions in `tools.json` and the design versions in `.github/scripts/designs.json`, and a bot creates a PR if there are updates to these files. The other CI runs when the bot creates the PR, which runs the full user flow.