##Motivation
This PR aims to improve the documentation for Windows users by providing a clear and beginner-friendly step-by-step installation guide.
Although MMAction2 already provides general installation instructions, setting up the environment on Windows can sometimes be confusing for new users. This addition helps improve accessibility and onboarding experience, especially for students and first-time contributors.

##Modification
Added a Windows Installation Guide section to README.md.
Included step-by-step instructions for:
Creating a Conda environment
Installing PyTorch (CPU and GPU options)
Installing MMAction2 dependencies
Verifying installation
No changes were made to the core source code.

##BC-breaking (Optional)
No.
This PR only modifies documentation and does not introduce any changes that affect backward compatibility.

##Use cases (Optional)
This documentation improvement helps:
Beginners installing MMAction2 on Windows systems
Students working on video understanding projects
Developers setting up development environments
New contributors who want clearer setup instructions

##Checklist
This PR only modifies documentation.
No core code logic was changed.
No unit tests are required.
Formatting was preserved and existing documentation style was maintained.