docs: add user guide installation page

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+
+# Installation
+
+Apache DataFusion Java has not yet published a release. Until the first
+release, the only way to use the library is to build from source.
+
+## Requirements
+
+- **JDK 17 or newer.** Set `JAVA_HOME` to point at it.
+- **Rust toolchain (stable).** Install via [rustup].
+
+[rustup]: https://rustup.rs/
+
+## Build from source
+
+```sh
+git clone https://github.com/apache/datafusion-java.git
+cd datafusion-java
+make test
+```
+
+`make test` compiles the native Rust crate, then runs the JUnit tests
+against it. The native library must be built before the JVM tests can
+run.
+
+The first build in a fresh checkout reaches out to
+`raw.githubusercontent.com` to fetch the DataFusion `.proto` files used to
+generate the `datafusion-proto` Java classes. Subsequent builds are
+offline; the `download-maven-plugin` cache under
+`~/.m2/repository/.cache/` satisfies them.
+
+For development workflow details — running individual tests, the TPC-H
+integration test data, code style, and how to update the underlying
+DataFusion version — see the [Contributor Guide](../contributor-guide/development.md).