Added changelog for 1.0.0 release

Jacob Wenger · Jacob Wenger · commit 094bef28bfb1 · 2016-05-25T16:45:36.000-07:00
diff --git a/changelog.txt b/changelog.txt
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+important - See the [Firebase migration guide](https://firebase.google.com/support/guides/firebase-web) for instructions on how to upgrade to the Firebase `3.x.x` SDK.
+feature - Upgraded the `firebase` dependency to `3.x.x`. ReactFire will continue to work with Firebase `2.x.x` as well.

Original file line number	Diff line number	Diff line change
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	`1`	+important - See the [Firebase migration guide](https://firebase.google.com/support/guides/firebase-web) for instructions on how to upgrade to the Firebase `3.x.x` SDK.
	`2`	+feature - Upgraded the `firebase` dependency to `3.x.x`. ReactFire will continue to work with Firebase `2.x.x` as well.