Expand introduction page.

docs/src/index.md

@@ -2,3 +2,20 @@
 
 HYPRE.jl is a Julia wrapper for the [HYPRE library](https://github.com/hypre-space/hypre),
 which provide parallel solvers for sparse linear systems.
+
+##### High level interface
+
+HYPRE.jl provide a high level interface to the HYPRE library. The goal of this interface is
+that the style and API should feel natural to most Julia programmers (it is "Julian"). In
+particular, you can use standard sparse matrices together with HYPRE's solvers through this
+interface.
+
+The high level interface does not (currently) provide access to all of HYPREs functionality,
+but it can easily be combined with the low level interface when necessary.
+
+##### Low level interface
+
+HYPRE.jl also provide a low level interface for interacting with HYPRE. The goal of this
+interface is to stay close to the HYPRE C API. In fact, this interface is automatically
+generated based on HYPRE's header files, so this API maps one-to-one with the C API, see
+[LibHYPRE C API](@ref) for more details.