docs: Add README for some newly added packages

[jennifer-shehane]

• Add README for runner-shared and runner-ct packages
• Update codeowners so e2e team is reequested for review on changes to runner-shared, .github, and scripts

Additional Details

We want to ensure all newly added packages are properly documented for open source contributors and our own team by adding a README that describes:

• What the package does.
• How to work within the package to watch files and run tests.
• Any other necessary info.

For a good example, see the reporter/README.md.

[cypress-bot]

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• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.
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[elevatebart]

Thank you @jennifer-shehane
This was indeed needed.

@lmiller1990 can you review the runner-shared readme

The rest is great with me.

[cypress]

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Test summary

18209 • 0 • 214 • 7 • 4

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Run details

Project	cypress
Status	Passed
Commit	028fa77
Started	Jul 12, 2021 2:17 PM
Ended	Jul 12, 2021 2:29 PM
Duration	11:22 💡
OS	Linux Debian - 10.8
Browser	Multiple

View run in Cypress Dashboard ➡️

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Flakiness

	cypress/integration/commands/net_stubbing_spec.ts		4
1	network stubbing > intercepting response > does not overwrite the json-related content-type header
2	... > automatically parses application/json response bodies
3	... > sets body to string if JSON is malformed
4	network stubbing > waiting and aliasing > can timeout waiting on a single request using "alias.request"

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[lmiller1990]

Should we also add a runner-ct code owner and respective GH team?

[jennifer-shehane]

@lmiller1990 Yah, was leaving that to the CT team to decide if you want to have your team pinged whenever the code is touched in the packages you 'own'. It's been working well for us because things opened by outside contributors or other teams is pretty obvious if it touches something we should look at without looking through all the files.