Create CHANGELOG.md (#75)

* Create CHANGELOG.md

Initial changelog based on https://firebase.google.com/support/release-notes/admin/go

* Update CHANGELOG.md

Author: teskey

CHANGELOG.md

@@ -0,0 +1,101 @@
+# Unreleased
+- 
+
+# v2.4.0
+
+### Initialization
+
+- [added] The [`firebase.NewApp()`](https://godoc.org/firebase.google.com/go#NewApp)
+  method can now be invoked without any arguments. This initializes an app
+  using Google Application Default Credentials, and
+  [`firebase.Config`](https://godoc.org/firebase.google.com/go#Config) loaded
+  from the `FIREBASE_CONFIG` environment variable.
+
+### Authentication
+
+- [changed] The user management operations in the `auth` package now uses the
+  [`identitytoolkit/v3`](https://google.golang.org/api/identitytoolkit/v3) library.
+- [changed] The `ProviderID` field on the
+  [`auth.UserRecord`](https://godoc.org/firebase.google.com/go/auth#UserRecord)
+  type is now set to the constant value `firebase`.
+
+# v2.3.0
+
+- [added] A new [`InstanceID`](https://godoc.org/firebase.google.com/go#App.InstanceID)
+  API that facilitates deleting instance IDs and associated user data from
+  Firebase projects.
+
+# v2.2.1
+
+### Authentication
+
+-  [changed] Adding the `X-Client-Version` to the headers in the API calls for
+  tracking API usage.
+
+# v2.2.0
+
+### Authentication
+
+- [added] A new user management API that supports querying and updating
+  user accounts associated with a Firebase project. This adds `GetUser()`,
+  `GetUserByEmail()`, `GetUserByPhoneNumber()`, `CreateUser()`, `UpdateUser()`,
+  `DeleteUser()`, `Users()` and `SetCustomUserClaims()` functions to the
+  [`auth.Client`](https://godoc.org/firebase.google.com/go/auth#Client) API.
+
+# v2.1.0
+
+- [added] A new [`Firestore` API](https://godoc.org/firebase.google.com/go#App.Firestore)
+  that enables access to [Cloud Firestore](/docs/firestore) databases.
+
+# v2.0.0
+
+- [added] A new [Cloud Storage API](https://godoc.org/firebase.google.com/go/storage)
+  that facilitates accessing Google Cloud Storage buckets using the
+  [`cloud.google.com/go/storage`](https://cloud.google.com/go/storage)
+  package.
+
+### Authentication
+
+- [changed] The [`Auth()`](https://godoc.org/firebase.google.com/go#App.Auth)
+  API now accepts a `Context` argument. This breaking
+  change enables passing different contexts to different services, instead
+  of using a single context per [`App`](https://godoc.org/firebase.google.com/go#App).
+
+# v1.0.2
+
+### Authentication
+
+- [changed] When deployed in the Google App Engine environment, the SDK can
+  now leverage the utilities provided by the
+  [App Engine SDK](https://cloud.google.com/appengine/docs/standard/go/reference)
+  to sign JWT tokens. As a result, it is now possible to initialize the Admin
+  SDK in App Engine without a service account JSON file, and still be able to
+  call [`CustomToken()`](https://godoc.org/firebase.google.com/go/auth#Client.CustomToken)
+  and [`CustomTokenWithClaims()`](https://godoc.org/firebase.google.com/go/auth#Client.CustomTokenWithClaims).
+
+# v1.0.1
+
+### Authentication
+
+- [changed] Now uses the client options provided during
+  [SDK initialization](https://godoc.org/firebase.google.com/go#NewApp) to
+  create the [`http.Client`](https://godoc.org/net/http#Client) that is used
+  to fetch public key certificates. This enables developers to use the ID token
+  verification feature in environments like Google App Engine by providing a
+  platform-specific `http.Client` using
+  [`option.WithHTTPClient()`](https://godoc.org/google.golang.org/api/option#WithHTTPClient).
+
+# v1.0.0
+
+- [added] Initial release of the Admin Go SDK. See
+  [Add the Firebase Admin SDK to your Server](/docs/admin/setup/) to get
+  started.
+- [added] You can configure the SDK to use service account credentials, user
+  credentials (refresh tokens), or Google Cloud application default credentials
+  to access your Firebase project.
+
+### Authentication
+
+- [added] The initial release includes the `CustomToken()`,
+  `CustomTokenWithClaims()`, and `VerifyIDToken()` functions for minting custom
+  authentication tokens and verifying Firebase ID tokens.