chore(README): add documentation badge
chore(README): add documentation badge

A badge for the readme.io documentation has been added.

The wording for the waffle board has also been changed to:
`project backlog` and the badge colour matches waffle.io.
thestonefox/VRTK,TMaloteaux/SteamVR_Unity_Toolkit,gpvigano/VRTK,thestonefox/SteamVR_Unity_Toolkit,gomez-addams/SteamVR_Unity_Toolkit,mattboy64/SteamVR_Unity_Toolkit,virror/VRTK,Fulby/VRTK,gpvigano/SteamVR_Unity_Toolkit,adamjweaver/VRTK,Innoactive/IA-unity-VR-toolkit-VRTK,red-horizon/VRTK