Update documentation

README.rst

@@ -89,6 +89,18 @@ For a non-virtual environment, you would follow a script like below:
     make -j
     ctest -V -R
 
+Eigen Testing
+*************
+
+Optionally, testing for Eigen support can be enabled.
+Providing the argument ``-DEigen_INCLUDE_DIR=<location of Eigen headers>`` to the CMake command line
+will enable the Eigen testing. Eigen must be installed on the system to properly implement this feature.
+To install Eigen, see `Eigen's documentation`_, or run the provided CI script that can be used locally
+`eigen.cmake` with the command `cmake -P .gitlab/ci/eigen.cmake` from the root of the source tree.
+This will download and unpack Eigen. The choice can then be made to install the Eigen headers to a favorite
+install directory via the instruction provided by the unpacked Eigen source, or, more simply, by providing the
+location of the unpacked source code to `Eigen_INCLUDE_DIR`.
+
 Preparing C++ code
 ------------------
 
@@ -860,4 +872,5 @@ Other Mentions
 .. _`pre-commit`: https://pre-commit.com/
 .. _`Pybind11's documentation`: https://pybind11.readthedocs.io/en/stable/classes.html#instance-and-static-fields
 .. _`Bazel Integration`: https://github.com/RobotLocomotion/drake/pull/14265
-.. _`CMake Integration`: https://gitlab.kitware.com/autopybind11/autopybind_skeleton/-/tree/drake
\ No newline at end of file
+.. _`CMake Integration`: https://gitlab.kitware.com/autopybind11/autopybind_skeleton/-/tree/drake
+.. _`Eigen Documentation`: https://eigen.tuxfamily.org/index.php?title=CMake
\ No newline at end of file

Tests/eigen/CMakeLists.txt

 cmake_minimum_required(VERSION 3.15)
-project(APB-EIGEN)
+project(APB-EIGEN CXX)
 
 add_executable(eigen-test eigen_test.cpp)
-message(WARNING ${Eigen_INCLUDE_DIR})
-target_include_directories(eigen-test PUBLIC ${Eigen_INCLUDE_DIR} )
+target_include_directories(eigen-test INTERFACE ${Eigen_INCLUDE_DIR} )