docs(README.md): replace slack badge with static badge
docs(README.md): replace slack badge with static badge

The dynamic slack badge no longer works as it was tied in to the
myslackin site, which is no longer used for the auto slack inviter.

There is no dynamic badge on the shields.io service so a simple
static badge is now being used.
thestonefox/SteamVR_Unity_Toolkit,Innoactive/IA-unity-VR-toolkit-VRTK,thestonefox/VRTK,gomez-addams/SteamVR_Unity_Toolkit