[Docs] Add migration.md and notice for default branch switching (#2407)

* add migration and notice

* fix link

* polish docstring

* polish docs

* polish docs

* minor changes

* add link of migration docs of MMEngine

* final check

README.md

@@ -49,6 +49,9 @@
 
 **News**:
 
+**We have renamed the branch `1.1`  to `main` and switched the default branch from `master` to `main`. We encourage
+users to migrate to the latest version, though it comes with some cost. Please refer to [Migration Guide](docs/en/migration.md) for more details.**
+
 **v1.1.0rc3** was released in 7/1/2023
 
 The compatibilities of models are broken due to the unification and simplification of coordinate systems after v1.0.0rc0. For now, most models are benchmarked with similar performance, though few models are still being benchmarked. In the following release, we will update all the model checkpoints and benchmarks. See more details in the [Changelog](docs/en/notes/changelog.md) and [Changelog-v1.0.x](docs/en/notes/changelog_v1.0.x.md).

README_zh-CN.md

@@ -47,6 +47,8 @@
 
 **新闻**：
 
+**我们将 `1.1` 分支重命名为 `main` 并将默认分支从 `master` 切换到 `main`。我们鼓励用户迁移到最新版本，请参考 [迁移指南](docs/en/migration.md) 以了解更多细节。**
+
 **v1.1.0rc3** 版本已经在 2023.1.7 发布。
 
 由于坐标系的统一和简化，模型的兼容性会受到影响。目前，大多数模型都以类似的性能对齐了精度，但仍有少数模型在进行基准测试。在接下来的版本中，我们将更新所有的模型权重文件和基准。您可以在[变更日志](docs/zh_cn/notes/changelog.md)和 [v1.0.x 版本变更日志](docs/zh_cn/notes/changelog_v1.0.x.md)中查看更多详细信息。

docs/en/index.rst

@@ -22,7 +22,7 @@ Welcome to MMDetection3D's documentation!
 
 .. toctree::
    :maxdepth: 1
-   :caption: Migration
+   :caption: Migrating from MMDetection3D 1.0
 
    migration.md
 

docs/en/migration.md

+Along with the release of OpenMMLab 2.0, MMDetection3D (namely MMDet3D) 1.1 made many significant changes, resulting in less redundant, more efficient code and a more consistent overall design. These changes break backward compatibility. Therefore, we prepared this migration guide to make the transition as smooth as possible so that all users can enjoy the productivity benefits of the new MMDet3D and the entire OpenMMLab 2.0 ecosystem.
+
+## Environment
+
+MMDet3D 1.1 depends on the new foundational library [MMEngine](https://github.com/open-mmlab/mmengine) for training deep learning models, and therefore has an entirely different dependency chain compared with MMDet3D 1.0. Even if you have a well-rounded MMDet3D 1.0 / 0.x environment before, you still need to create a new Python environment for MMDet3D 1.1. We provide a detailed [installation guide](./get_started.md) for reference.
+
+The configuration files in our new version have a lot of modifications because of the differences between MMCV 1.x and MMEngine. The guides for migration from MMCV to MMEngine can be seen [here](https://github.com/open-mmlab/mmengine/tree/main/docs/en/migration).
+
+## Dataset
+
+You should update the annotation files generated in the 1.0 version since some key words and structures of annotation in MMDet3D 1.1 have changed. Taking KITTI as an example, the update script is as follows:
+
+```python
+python tools/dataset_converters/update_infos_to_v2.py
+        --dataset kitti
+        --pkl-path ./data/kitti/kitti_infos_train.pkl
+        --out-dir ./kitti_v2/
+```
+
+If your annotation files are generated in the 0.x version, you should first update them to 1.0 version using this [script](../../tools/update_data_coords.py). Alternatively, you can re-generate annotation files from scratch using this [script](../../tools/create_data.py).
+
+## Model
+
+MMDet3D 1.1 supports loading weights trained on the old version (1.0 version). For models that are important or frequently used, we have thoroughly verified their precisions in the 1.1 version. Especially for some models that may experience potential performance drop or training bugs in the old version, such as [centerpoint](https://github.com/open-mmlab/mmdetection3d/issues/2390), we have checked them and ensured the right precision in the new version. If you encounter any problem, please feel free to raise an [issue](https://github.com/open-mmlab/mmdetection3d/issues). Additionally, we have added some of the latest SOTA methods in our [package](../../configs/) and [projects](../../projects/), making MMDet3D 1.1 a highly recommended choice for implementing your project.