add readme stuff

README.md

@@ -4,3 +4,18 @@
 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://svretina.github.io/Zerilli.jl/dev/)
 [![Build Status](https://github.com/svretina/Zerilli.jl/actions/workflows/CI.yml/badge.svg?branch=master)](https://github.com/svretina/Zerilli.jl/actions/workflows/CI.yml?query=branch%3Amaster)
 [![Coverage](https://codecov.io/gh/svretina/Zerilli.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/svretina/Zerilli.jl)
+
+This package is evolving the radial part of the Zerilli equation to calculate 
+the evolution of black hole pertubations. The equations are put into first order
+formulation and boundary conditions are applied via the characteristic approach. Boundary conditions
+can be either reflective or radiative. It uses 2nd order stencils for the discretization
+in the interior and 1st order at the boundary points, so that we employ stability proofs from 
+the Summation By Parts (SBP) property. 
+
+
+## TODO
+- write some docs 
+- write correct initial data
+- write potential
+- put correct equations with the potential term
+- (add threading? not needed right now)