add documentation to readme

Author: ArnoStrouwen

README.md

@@ -2,6 +2,15 @@
 
 **SymbolicNumericIntegration.jl** is a hybrid symbolic/numerical integration package that works on the [Julia Symbolics](https://github.com/JuliaSymbolics/Symbolics.jl) expressions.
 
+## Tutorials and Documentation
+
+For information on using the package,
+[see the stable documentation](https://symbolicnumericintegration.sciml.ai/stable/). Use the
+[in-development documentation](https://symbolicnumericintegration.sciml.ai/dev/) for the version of
+the documentation, which contains the unreleased features.
+
+## Example
+
 ```julia
 using Symbolics
 using SymbolicNumericIntegration