docs: Create packagist.md

Describing how Packagist is updated.

docs/packagist.md

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+# Content Build in Packagist
+[Packagist](https://packagist.org/) is the main public package library, similar to NPM for Javascript or Gems for Ruby. [Composer](https://getcomposer.org/) is used to manage PHP packages for a given application, similar to the `npm` or `gem` applications.
+
+[Content Build is published to Packagist as a public package](https://packagist.org/packages/va-gov/content-build). This enables PHP applications to pull in Content Build via its Composer document as part of its build process.
+
+## Updating Content Build on Packagist
+Content Build on Packagist is updated automatically via webhook; see [Packagist's documentation for Github webhook integration](https://packagist.org/about), and Content Build's own Github settings for the specifics of the webhook.