Add UPGRADE instructions

Author: GromNaN

UPGRADE-2.0.md

@@ -0,0 +1,28 @@
+UPGRADE FROM 1.x to 2.0
+========================
+
+GridFS
+------
+
+ * The `md5` is no longer calculated when a file is uploaded to GridFS.
+   Applications that require a file digest should implement it outside GridFS
+   and store in metadata.
+
+   ```php
+   $hash = hash_file('sha256', $filename);
+   $bucket->openDownloadStream($fileId, ['metadata' => ['hash' => $hash]]);
+   ```
+
+ * The fields `contentType` and `aliases` are no longer stored in the `files`
+   collection. Applications that require this information should store it in
+   metadata.
+
+   **Before:**
+   ```php
+   $bucket->openDownloadStream($fileId, ['contentType' => 'image/png']);
+   ```
+
+   **After:**
+   ```php
+   $bucket->openDownloadStream($fileId, ['metadata' => ['contentType' => 'image/png']]);
+   ```