Add indicator constraint to README (#445)

Author: odow

README.md

@@ -97,6 +97,7 @@ List of supported constraint types:
  * [`MOI.VectorOfVariables`](@ref) in [`MOI.SOS1{Float64}`](@ref)
  * [`MOI.VectorOfVariables`](@ref) in [`MOI.SOS2{Float64}`](@ref)
  * [`MOI.VectorOfVariables`](@ref) in [`MOI.SecondOrderCone`](@ref)
+ * [`MOI.VectorAffineFunction`](@ref) in [`MOI.Indicator`](@ref)
 
 List of supported model attributes: