Add v3.0.0 release notes

[jamsinclair]

Thanks for your work on the library @matthew-andrews!

I think many of us will be seeing this update coming through automated Pull Requests (e.g. dependabot) and will want to know the changes, especially for a breaking change. It'd be great if we could add a description to your v3.0.0 release.

I've tried writing a summary below based on https://github.com/matthew-andrews/isomorphic-fetch/compare/v2.2.1...v3.0.0, please feel free to update as you see fit 😸

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Breaking Changes:

• Bumps node-fetch and whatwg-fetch dependencies to their latest major versions for security fixes and updated features.
• As a result of dependency upgrades Node.js 0.10.x and 0.12.x support is dropped

[shanytc]

Finally! thank you!

[0rvar]

Great summary, thanks @jamsinclair!

@matthew-andrews could you update the release with this summary and mark it as latest?