Update README to include documentation for sinpi, cospi, etc.

README.md

@@ -15,3 +15,10 @@ tau == τ == 2*pi  # => true
 typeof(tau)       # => MathConst{:τ} (constructor with 1 method)
 ```
 
+The tau variants of `sinpi`, `cospi`, and `mod2pi` are also defined:
+
+```julia
+sintau(1//4) # => 1.0
+costau(1//2) # => -1.0
+```
+