Add migrate_from_folder in readme.

README.md

@@ -2,10 +2,10 @@
 
 ```python
 import torch
-from migrate_ckpt import Migration, migrate_ckpt
+from migrate_ckpt import CkptType, Migration, migrate_ckpt
 
 
-def update_some_keys_callback(ckpt):
+def update_some_keys_callback(ckpt: CkptType) -> CkptType:
     """
     Define a callback that takes a checkpoints and updates it.
     """
@@ -36,3 +36,37 @@ Note: the list of migration to perform is determined by the last done migration.
 Missed migration in between will never be done.
 For example, if migrations to do are ["0", "1", "2"] and model has already had migration
 "1", only "2" will be done, but not "0".
+
+## Store migrations in a folder
+
+It migth be convenient to store all your migrations in a specific folder, and execute
+migrations from this folder. You can do this with `migrate_from_folder` function.
+
+For example, create a `migrations` folder with this files (loaded in alphabetical order):
+```
+0_initial_migration.py
+1_second_migration.py
+2_this_comes_next.py
+```
+
+In each migration file, you need to define a function:
+```python
+from migrate_ckpt import CkptType
+
+
+def handle(ckpt: CkptType) -> CkptType:
+    # do stuff with ckpt
+    return ckpt
+```
+
+
+You can then execute all migrations in the folder with:
+```python
+import torch
+
+from migrate_ckpt import migrate_from_folder
+
+ckpt, done_migrations = migrate_from_folder(
+    torch.load("/path/to/some/checkpoint.ckpt"), "path/to/migration/folder"
+)
+```