Add instructions for dev build

[helin24]

We have instructions for using the dev build in the Flutter plugin but not here. I wanted a spot in README to point users to when we suggest they try out a fix.

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Review the contribution guidelines below:

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
• I've included the required information in the description above.
• I've updated CHANGELOG.md if appropriate (i.e. user-facing change)

Contribution guidelines:

• See the Flutter organization contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Dart contributions to our repos should follow the Dart style guide and use
  dart format.
• Java and Kotlin contributions should strive to follow Java and Kotlin best
  practices (discussion).

[gemini-code-assist]

Code Review

This pull request adds instructions to the README on how to install development builds of the Dart plugin in IntelliJ IDEA. The review feedback suggests adding a corresponding entry to the CHANGELOG.md file as required by the style guide, and generalizing the location of the gear icon in the instructions to support different IntelliJ versions.