[pigeon] add null safety guide to readme

[fzyzcjy]

Replace this paragraph with a description of what this PR is changing or adding, and why.

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

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

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[gaaclarke]

I tried to rebase this for you but I don't seem to have permissions. I also tried tweaking the text:

## Null Safety (NNBD)

In order to generate null-safe code run the command line with the extra argument
`--dart_null_safety`. For example:
`flutter pub run pigeon --input ./pigeons/messages.dart --dart_null_safety`.

[fzyzcjy]

@gaaclarke Hmm so what can I do?

[gaaclarke]

You need to rebase onto master or give me permissions to push to your PR.

Rebasing will be something like this (its a bit weird because your branch is named master):

git fetch origin
git rebase -i <UPSTREAM REMOTE>/master
# handle conflicts
git add README.md # commit resolution to conflict
git rebase --continue
git push -f <YOUR REMOTE>

[gaaclarke]

Or if you don't want to deal with it, i can duplicate the change in a different PR.

[fzyzcjy]

@gaaclarke Sure. Let me rebase it.

[fzyzcjy]

@gaaclarke Done