chore(README): remove kickstarter link
chore(README): remove kickstarter link

The Kickstarter campaign has finished and the link has been removed.
TMaloteaux/SteamVR_Unity_Toolkit,mattboy64/SteamVR_Unity_Toolkit,Fulby/VRTK,gpvigano/SteamVR_Unity_Toolkit,adamjweaver/VRTK,virror/VRTK,red-horizon/VRTK,gpvigano/VRTK,thestonefox/SteamVR_Unity_Toolkit,thestonefox/VRTK,Innoactive/IA-unity-VR-toolkit-VRTK,gomez-addams/SteamVR_Unity_Toolkit