add docs of how to upgrade to latest relay version

[sibelius]

• update package.json to latest relay version
• run yarn sync-fixtures, so fixtures are updated
• run yarn test

what else?

[alloy]

Manually take a look at the Flow tests and perhaps also scan through the commits to look for anything related to (Flow) types.

[alloy]

I wonder if there’s a service that can just file an issue on this repo anytime one of the files related to type emission is updated upstream? 🤔

[zephraph]

If not, we can build it. We have the technology. 😄

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