Rust: Add build instructions to the Readme
Rust bindings for libelektra.
+## Build
+
+To build the bindings explicitly as part of the elektra build process, we pass the option `-DBINDINGS="rust"` to cmake. After [building libelektra](../../../doc/COMPILE.md), we can now add the `elektra` crate to the dependencies. The exact paths depends on your system.
+
+```toml
+[dependencies]
+elektra = { version = "0.9.0", path = "~/git/libelektra/build/src/bindings/rust/elektra" }
+# Directly depending on elektra-sys is only needed if you need to use the raw bindings
+elektra-sys = { version = "0.9.0", path = "~/git/libelektra/build/src/bindings/rust/elektra-sys" }
+```
+
## Usage
### Raw Bindings
