docs(ivy): Add README to indexer module

[ayazhafiz]

Describe the indexer module for Angular compiler developers. Include
scope of analysis provided by the module and the indexers it targets as
first-party.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

No information about why the indexer module exists.

Issue Number: N/A

What is the new behavior?

Explain why the indexer module exists.

Does this PR introduce a breaking change?

• Yes
• No

Other information

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