docs(expo): Outline manual dependency sync process

[bengourley]

Goal

Document the process so that engineers without prior knowledge of the project's nuances can successfully keep Expo deps in sync.

Design

Keeping it simple and local to the project, and subject to the same review process for changes/updates.

Changeset

Added a CONTRIBUTING.md to the @bugsnag/expo package.

Testing

N/A

[bugsnagbot]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	40.57 kB	12.52 kB
After	40.57 kB	12.52 kB
±	No change	No change

Generated by 🚫 dangerJS against f375f69

[imjoehaines]

I initially read this as "When the version of the bundled native modules changes, the notifier might be incompatible with previous Expo SDKs", when I think it means "When the version of the bundled native modules changes, the notifier will be incompatible with previous Expo SDKs"

Something like this might be clearer?

Suggested change

	When the version of the bundled native modules changes, making the notifier incompatible with previous Expo SDKs we need to update the CLI using the established pattern. See `packages/expo-cli/commands/install.js` for details.
	When the version of the bundled native modules changes the notifier will be incompatible with previous Expo SDKs. To prevent installing the conflicting versions, we need to update the CLI using the established pattern in [`packages/expo-cli/commands/install.js`](../expo-cli/commands/install.js).

[bengourley]

Mergining in spite of CI failure. This was due to an issue that is now fixed in #1116. This PR only contains a markdown file.