Added note in documentation regarding make install (#1801)

* Added note regarding make install

docs/compiling.rst

@@ -149,6 +149,18 @@ See the `Config file`_ docstring for details of relevant CMake variables.
     find_package(pybind11 REQUIRED)
     find_package(pybind11 REQUIRED)
     pybind11_add_module(example example.cpp)
     pybind11_add_module(example example.cpp)
 
 
+Note that ``find_package(pybind11)`` will only work correctly if pybind11
+has been correctly installed on the system, e. g. after downloading or cloning
+the pybind11 repository  :
+
+.. code-block:: bash
+
+    cd pybind11
+    mkdir build
+    cd build
+    cmake ..
+    make install
+
 Once detected, the aforementioned ``pybind11_add_module`` can be employed as
 Once detected, the aforementioned ``pybind11_add_module`` can be employed as
 before. The function usage and configuration variables are identical no matter
 before. The function usage and configuration variables are identical no matter
 if pybind11 is added as a subdirectory or found as an installed package. You
 if pybind11 is added as a subdirectory or found as an installed package. You