Update library usage documentation

LIBRARY_USAGE.md

@@ -88,6 +88,59 @@ Simply include the source files directly in your project:
    gcc your_project.c -lnostr_core -lm
    ```
 
+## Building the Library
+
+### Using the Provided Build Script
+
+The library includes an automated build script that handles all dependencies and creates a self-contained static library.
+
+**IMPORTANT**: The build script must be run from within the `nostr_core_lib` directory:
+
+```bash
+# Correct usage:
+cd nostr_core_lib
+./build.sh
+
+# If you need to use it from your project directory:
+cd nostr_core_lib
+./build.sh
+cd ..
+gcc your_app.c nostr_core_lib/libnostr_core_x64.a -lz -ldl -lpthread -lm -o your_app
+```
+
+**Common Error**: Running `./nostr_core_lib/build.sh` from outside the library directory will fail with:
+```
+[ERROR] Build script must be run from the nostr_core_lib directory
+```
+
+### Build Script Options
+
+```bash
+# Auto-detect NIPs from your source code
+./build.sh
+
+# Force specific NIPs
+./build.sh --nips=1,6,19
+
+# Build all available NIPs
+./build.sh --nips=all
+
+# Build for specific architecture
+./build.sh arm64
+
+# Build with tests
+./build.sh --tests
+
+# Get help
+./build.sh --help
+```
+
+The build script automatically:
+- Scans your `.c` files for `#include "nip*.h"` statements
+- Compiles only the needed NIPs
+- Creates a self-contained static library (`libnostr_core_x64.a`)
+- Includes all dependencies (OpenSSL, curl, secp256k1, cJSON)
+
 ## Basic Usage Example
 
 ```c