Update README with some useful information. (#1)

Co-authored-by: Holger Rapp <[email protected]>

README.md

@@ -1,2 +1,21 @@
-# zvt
-Rust implementation of the ZVT protocol
+# ZVT
+
+This is an implementation of the ECR ZVT Protocoll used in payment terminals
+across Europe. It follows the ECR-Interface / ZVT - Protocol specification
+defined by the Verband der Terminal Hersteller e.V.. Currently, we follow
+Revision 13.11.
+
+The specification can be downloaded
+[here](https://www.terminalhersteller.de/downloads.aspx) and the code, as well
+as the documentation references sections in this document.
+
+The code also implements extensions defined by
+[Feig](https://www.feig-payment.de/), because this is the Terminal that Qwello
+uses in production in most of our charging stations.
+
+## Getting started
+
+Start by looking in [`status.rs`](zvt/examples/status.rs) for a typical way of
+interfacing with a terminal. A useful standalone tool is
+[`feig_update.rs`](zvt/examples/feig_update.rs) which we use in production to
+update the Firmware of our cVEND plug terminals.