chore(README): add link to license file
chore(README): add link to license file

The License text in the readme now points to the `LICENSE` file
included within the repo.
gpvigano/SteamVR_Unity_Toolkit,gpvigano/VRTK,jcrombez/SteamVR_Unity_Toolkit,Innoactive/IA-unity-VR-toolkit-VRTK,gomez-addams/SteamVR_Unity_Toolkit,virror/VRTK,red-horizon/VRTK,adamjweaver/VRTK,TMaloteaux/SteamVR_Unity_Toolkit,mattboy64/SteamVR_Unity_Toolkit,pargee/SteamVR_Unity_Toolkit,Blueteak/SteamVR_Unity_Toolkit,thacio/SteamVR_Unity_Toolkit,mimminito/SteamVR_Unity_Toolkit,Fulby/VRTK,nmccarty/htvr,thestonefox/VRTK,thestonefox/SteamVR_Unity_Toolkit,bengolds/SteamVR_Unity_Toolkit,mbbmbbmm/SteamVR_Unity_Toolkit,phr00t/SteamVR_Unity_Toolkit