Document build dependencies

This patch adds a section with the build dependencies to the readme.

README.md

@@ -70,6 +70,21 @@ Download the latest compiled [alpha release](https://github.com/Nitrokey/nitroke
 Plug your Nitrokey 3 and use [nitropy](https://docs.nitrokey.com/software/nitropy/) to install it with 
 `nitropy nk3 update <path/to/release/file>`
 
+## Build Dependencies
+
+`opcard` has these build dependencies:
+- clang
+- libpcsclite
+- nettle
+- pkg-config
+
+To run the tests, you also need these tools:
+- gnupg
+- scdaemon
+- vsmartcard (vpcd)
+
+See the [CI Dockerfile](./ci/Dockerfile) for all steps to set up the build environment on Debian-based distributions.
+
 ## Minimum Supported Rust Version (MSRV)
 
 The minimum supported Rust version (MSRV) for this crate is the most recent stable Rust release.