Add Data directory download instructions

Data/README.md

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+# Robotwin ACT Data Archive
+
+This directory contains the split archive for `robotwin-act-data.tar.gz`.
+
+## Files
+
+Download all seven parts:
+
+```text
+robotwin-act-data.tar.gz.part001
+robotwin-act-data.tar.gz.part002
+robotwin-act-data.tar.gz.part003
+robotwin-act-data.tar.gz.part004
+robotwin-act-data.tar.gz.part005
+robotwin-act-data.tar.gz.part006
+robotwin-act-data.tar.gz.part007
+```
+
+The merged archive should be:
+
+```text
+robotwin-act-data.tar.gz
+```
+
+Expected merged size:
+
+```text
+128,050,721,979 bytes
+```
+
+Expected SHA256:
+
+```text
+6B10200A8800B70700084F9437175151886D349B1270A4A659172F0C548A6284
+```
+
+## Download
+
+Install the Hugging Face CLI, then run:
+
+```bash
+hf download MetaAct/robotwin-act --repo-type dataset --include "Data/robotwin-act-data.tar.gz.part*"
+```
+
+## Merge
+
+After downloading, merge the parts in order.
+
+Windows CMD:
+
+```cmd
+copy /b Data\robotwin-act-data.tar.gz.part001+Data\robotwin-act-data.tar.gz.part002+Data\robotwin-act-data.tar.gz.part003+Data\robotwin-act-data.tar.gz.part004+Data\robotwin-act-data.tar.gz.part005+Data\robotwin-act-data.tar.gz.part006+Data\robotwin-act-data.tar.gz.part007 robotwin-act-data.tar.gz
+```
+
+Linux/macOS:
+
+```bash
+cat Data/robotwin-act-data.tar.gz.part* > robotwin-act-data.tar.gz
+```
+
+## Verify
+
+Windows PowerShell:
+
+```powershell
+Get-FileHash -Algorithm SHA256 .\robotwin-act-data.tar.gz
+```
+
+Linux/macOS:
+
+```bash
+sha256sum robotwin-act-data.tar.gz
+```
+
+The checksum must match:
+
+```text
+6B10200A8800B70700084F9437175151886D349B1270A4A659172F0C548A6284
+```
+