[Pigeon] Update README.md

[manuelperez96]

Add extra details in iOS steps.

I found that iOS steps are very confuse, so I've added a extra description to facilitate the configuration

Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.

List which issues are fixed by this PR. You must list at least one issue.

Pre-launch Checklist

• [X] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [X] I read the Tree Hygiene page, which explains my responsibilities.
• [X] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• [X] I signed the CLA.
• [X] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [ ] I linked to at least one issue that this PR fixes in the description above.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [ ] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or this PR is test-exempt.
• [X] All existing and new tests are passing.

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[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

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[stuartmorgan-g]

• [x] I signed the CLA.

The CLA check isn't passing, so if you have signed the CLA it was with a different email address than you used for the commit. Marking as a draft until you've had a chance to resolve that, since we can't review a PR without a valid CLA.

[stuartmorgan-g]

Unfortunately we’re unable to accept PRs without a signed CLA. If you sign the CLA in the future, please feel free to submit a new PR.