Add a script to migrate files between storage engines

Summary: Quora requested this (moving to S3) but it's also clearly a good idea.

Test Plan:
Ran with various valid/invalid options to test options. Error/sanity checking seemed OK.

Migrated individual local files.

Migrated all my local files back and forth between engines several times.

Uploaded some new files.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1950

Differential Revision: https://secure.phabricator.com/D3808

src/docs/configuration/configuring_file_storage.diviner

@@ -80,6 +80,22 @@ Technical Documentation}.
 You can test that things are correctly configured by going to the Files
 application (##/file/##) and uploading files.
 
+= Migrating Files Beteeen Engines =
+
+If you want to move files between storage engines, you can use the `bin/files`
+script to perform migrations. For example, suppose you previously used MySQL but
+recently set up S3 and want to migrate all your files there. First, migrate one
+file to make sure things work:
+
+  phabricator/ $ ./bin/files migrate --engine amazon-s3 F12345
+
+If that works properly, you can then migrate everything:
+
+  phabricator/ $ ./bin/files migrate --engine amazon-s3 --all
+
+You can use `--dry-run` to show which migrations would be performed without
+taking any action. Run `bin/files help` for more options and information.
+
 = Next Steps =
 
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