Update objective: Explicitly mention native and protondb whitelisted

The project overview markdown doesn't make it absolutely clear that the list is for native and protondb whitelisted software. It is kind of mentioned with "Also check out the VR tag on ProtonDB for games not listed here that may work with Proton".

With the recent #15 (closed) and #16 the confusion can be seen.

Maybe

Objective: Investigate software such as these native games on Steam to find whether they actually support virtual reality (VR) on Linux. In addition, Protondb whitelisted software will also be tracked. Please post what issues you have here.

Another option is to keep another list of gold/platinum proton db titles in anticipation of them being whitelisted.

Just a suggestion.