update docs

docs/index.md

@@ -1,7 +1,7 @@
 Taxcalc-Payroll
 ==============
 
-## What is Taxcalc-Payroll?
+## Brief
 
 Taxcalc-Payroll is a microsimulation model for static analysis of
 USA federal payroll taxes.
@@ -44,6 +44,15 @@ Taxcalc-Payroll with results from other tax models.
 The cross-model validation work with NBER's TAXSIM model is described
 [here](https://github.com/PSLmodels/Tax-Calculator/tree/master/taxcalc/validation).
 
+## A parallel project with Tax-Calculator
+
+Taxcalc-Payroll focuses on payroll tax analysis. It is a parallel project with Tax-Calculator 
+which aims to be focusing more on income tax analysis. Taxcalc-Payroll inherits the infrastructure 
+from Tax-Calculator. Two models are highly integrated. Users can do both income tax and payroll tax
+ analysis in either of the model
+
+Please read the details at {doc}`about/history`.
+
 ## Latest release
 
 {doc}`0.1.0 (2023-07-28) <about/releases>`