README: Add supported platforms, build instructions

Signed-off-by: Guillermo Rodríguez <grodriguez@ingelabs.com>

Author: guillerodriguez

README.md

@@ -1,5 +1,102 @@
-# Overview
+# GNU Classpath
 
-This is a fork of the original GNU Classpath git repository.
+GNU Classpath is a set of core class libraries for use with Java Virtual Machines, providing roughly Java 1.6 (Java SE 6) API compatibility.
+
+## Why GNU Classpath
+
+GNU Classpath provides a lightweight alternative to OpenJDK for environments where a full JDK is unnecessary. Combined with a compact VM such as [JamVM](https://github.com/ingelabs/jamvm), it is well suited for embedded and resource-constrained devices, offering:
+
+- Smaller flash and RAM footprint
+- Lower startup overhead
+- Minimal impact on runtime performance for typical workloads
+
+This makes it a practical choice when deploying Java applications on hardware where the size and startup cost of a full OpenJDK installation is prohibitive.
+
+## Supported platforms
+
+The primary targets are Linux embedded systems. The following architectures are supported:
+
+- Linux / 32-bit ARM
+- Linux / 64-bit ARM (aarch64)
+- Linux / x86_64
+
+macOS with Apple Silicon is also supported for development (not as a target).
+
+Other platforms supported by the build system may work but are not actively tested.
+
+## Building GNU Classpath
+
+### Prerequisites
+
+A Java 8 JDK is required to compile the class library.
+
+For Linux, Temurin or Zulu are good choices. For macOS, Zulu is recommended (Temurin is not available for Apple Silicon).
+
+Additional prerequisites for a typical headless configuration:
+
+**Linux (Debian/Ubuntu):**
+
+```sh
+sudo apt-get install build-essential autoconf automake libtool gettext texinfo
+```
+
+**macOS:**
+
+```sh
+brew install autoconf automake libtool gettext texinfo
+```
+
+You may need to link the gettext m4 macros into the aclocal directory (see [Homebrew#53192](https://github.com/Homebrew/homebrew-core/issues/53192)):
+
+```sh
+ln -sf "$(brew --prefix gettext)/share/gettext/m4/"*.m4 "$(aclocal --print-ac-dir)/"
+```
+
+### Build
+
+```sh
+autoreconf -iv
+./configure \
+  --disable-gtk-peer \
+  --disable-gconf-peer \
+  --enable-default-preferences-peer=file \
+  --disable-examples \
+  --disable-tools \
+  --disable-gjdoc
+make
+sudo make install
+```
+
+### Cross-compilation
+
+When building for embedded targets, a cross-compilation toolchain is needed.
+
+For building complete embedded Linux images, consider using tools such as [Buildroot](https://buildroot.org/) or [ptxdist](https://www.ptxdist.org/).
+
+## Building applications
+
+To compile a Java application against GNU Classpath, use `-bootclasspath` to point to the Classpath library classes, for example:
+
+```sh
+javac -source 1.6 -target 1.6 -bootclasspath /usr/local/classpath/share/classpath/glibj.zip HelloWorld.java
+```
+
+This ensures the compiler only allows APIs provided by GNU Classpath. Any use of unsupported APIs will result in a compile-time error.
+
+## Running applications
+
+To run an application with [JamVM](https://github.com/ingelabs/jamvm):
+
+```sh
+jamvm HelloWorld
+```
+
+JamVM automatically locates the GNU Classpath library classes from its installation prefix.
+
+## History
+
+This project is a fork of the original GNU Classpath project, which is no longer actively maintained (last release: 0.99, March 2012).
 
 The original project site can be found here: https://www.gnu.org/software/classpath/
+
+This fork continues development with a focus on embedded Linux targets.