README.md: Add description how to download the model files

[norihiro]

Description

Add a description how to prepare and install model files, as suggested in #137.

How Has This Been Tested?

I will check it later.

General checklist

• [ ] The commit is reviewed by yourself.
• [ ] The code is tested.
• [x] Document is up to date or not necessary to be changed.
• [x] The commit is compatible with repository's license.