Clarify which releases are supported

[neuromancer]

There are a few Death Rally releases:

• Shareware version
• Retail version
• Steam release

It will be useful to clarify in the README which one are compatible with the current state of the code.

[BSzili]

The shareware probably won't work due to the missing content, but as of commit 983cc9c the Steam release does in addition to the original retail version.