Create README.md explaining shims in src/MaxText

Author: bvandermoon

src/MaxText/README.md

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+<!--
+ # Copyright 2023–2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+ -->
+
+# src/MaxText
+
+The contents of `src/MaxText` have moved to `src/maxtext` as part of a larger 
+[restructuring effort in MaxText](https://github.com/AI-Hypercomputer/maxtext/blob/2790ed289c0c4cb704645d5d2ab91da26711b891/RESTRUCTURE.md).
+This directory only contains shim files to temporarily support legacy commands like `python3 -m MaxText.train ...`.
+These legacy commands are now deprecated and will be removed soon. Please migrate your existing commands and avoid using
+legacy ones. The new command locations can be found at:
+
+* `MaxText.decode` → `maxtext.inference.decode`
+* `MaxText.distillation.train_distill` → `maxtext.trainers.post_train.distillation.train_distill`
+* `MaxText.maxengine_server` → `maxtext.inference.maxengine.maxengine_server`
+* `MaxText.rl.evaluate_rl` → `maxtext.trainers.post_train.rl.evaluate_rl`
+* `MaxText.rl.train_rl` → `maxtext.trainers.post_train.rl.train_rl`
+* `MaxText.sft.sft_trainer` → `maxtext.trainers.post_train.sft.train_sft`
+* `MaxText.train` → `maxtext.trainers.pre_train.train`
+* `MaxText.train_compile` → `maxtext.trainers.pre_train.train_compile`
+* `MaxText.train_tokenizer` → `maxtext.trainers.tokenizer.train_tokenizer`