Changelog for v2.0.0 (#212)

Author: laurenzlong

changelog.txt

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+important - [breaking change] For Firestore-triggered functions, `snapshot.createTime`, `snapshot.updateTime`, `snapshot.readTime`, and any timestamp values in `snapshot.data()` are now [Firestore Timestamp](https://cloud.google.com/nodejs/docs/reference/firestore/0.15.x/Timestamp#properties) objects.
+feature - Support Node.js 8 runtime. To deploy your functions to Node.js 8, add `"engines": {"node": "8"}` to `functions/package.json`. You will need `firebase-tools` >=v4.0.0. 
+feature - Support selection of regions for your functions through the `functions.region` method. Learn more in the [Firebase Documentation](https://firebase.google.com/docs/functions/locations). You will need `firebase-tools` >=v4.0.0.
+feature - Support configuration of timeout and memory allocation through the `functions.runWith` method. Learn more in the [Firebase Documentation](https://firebase.google.com/docs/functions/manage-functions#set_timeout_and_memory_allocation). You will need `firebase-tools` >=v4.0.0.