docs(README.md): add v4 beta text to readme for clarity
docs(README.md): add v4 beta text to readme for clarity

The v4 beta text has been added to the title in the Readme just to
make it clear that the new branch is the v4 beta.

thestonefox/SteamVR_Unity_Toolkit,thestonefox/VRTK,gomez-addams/SteamVR_Unity_Toolkit