Add a separate readme for the library

schunk_egu_egk_gripper_library/README.md

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+# Schunk EGU/EGK Gripper Library
+
+This is the C++ driver library for SCHUNK EGU/EGK grippers.
+It implements the core functionality of the communication for usage in non-ROS2 C++ frameworks.
+
+## Standalone build and usage
+
+Get the `schunk_egu_egk_gripper` package following [these instructions](../README.md#build-and-install).
+Navigate into the `schunk_egu_egk_gripper_library` sub package and call
+
+```bash
+mkdir build && cd "$_"
+cmake ..
+make
+```
+to build the library with plain *CMake*.
+You can then include this library in the *CMakeLists.txt* of your application with the usual functionality:
+```cmake
+cmake_minimum_required(VERSION 3.22)
+project(your_application)
+
+# Find the library as a dependency
+find_package(schunk_egu_egk_gripper_library REQUIRED)
+
+# Link your application against the library
+target_link_libraries(your_application
+  Schunk::schunk_egu_egk_gripper_library
+)
+
+```