Add README on basic CMake usage.

README.cmake

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+Getting started with CMake
+
+(These instructions apply to Unix-like systems, including Cygwin and Mac. To
+build using Visual Studio or some other IDE supported by CMake, most of the
+information below still applies)
+
+Always compile in a separate directory to the code. For example, if the
+code (eg, from Git) is at /home/curt/projects/flightgear, you might create
+/home/curt/projects/fgbuild. Change into the new directory, and run
+
+    cmake ../flightger
+    
+To generate standard Unix Makefiles in fgbuild.
+
+Probably you want to specify an install prefix:
+
+    cmake ../flightgear -DCMAKE_INSTALL_PREFIX=/usr
+
+Note the install prefix is automatically searched for required libraries
+and header files, so if you install PLIB, OpenSceneGraph and SimGear to the
+same prefix, most configuration options are unnecessary.
+
+To specify that a particular dependency is in a non-standard location, most
+libraries support an environment variable - eg PLIBDIR or OSG_ROOT - to
+allow precise selection.
+
+By default, we select a release build. To create a debug build, use
+
+    cmake ../flightgear -DCMAKE_BUILD_TYPE=Debug
+    
+(or MinSizeRel, or RelWithDbg)
+
+Debug builds will automatically use corresponding debug builds of required
+libraries, if they are available. For example you can install debug builds of
+SimGear and OpenSceneGraph, and a debug FlightGear build will use them.
+
+(Debug builds of libraries have the 'd' suffix by default)
+
+Note most IDE projects (eg Xcode and Visual Studio) support building all the
+build types from the same project, so you can omit the CMAKE_BUILD_TYPE option
+when running cmake, and simply pick the build configuration as normal in the
+IDE.
+
+It's common to have several build directories with different build
+configurations, eg
+
+    /home/curt/projects/flightgear (the git clone)
+    /home/curt/projects/fgdebug
+    /home/curt/projects/fgrelease
+    /home/curt/projects/fg-with-svn-osg
+
+To set an optional feature, do
+
+    cmake ../flightgear -DFEATURE_NAME=ON
+
+The easiest way to see the features that can be configured, is by running one
+of the cmake GUIs - either ncurses or graphical. You can also check the root
+CMakeLists.txt file.
+
+Build Targets
+
+For a Unix makefile build, 'make dist', 'make uninstall' and 'make test' are
+all available and should work as expected. 'make clean' is also as normal,
+but there is *no* 'make distclean' target. The equivalent is to completely
+remove your build directory, and start with a fresh one.
+
+Adding new files to the build
+
+Add source files to the SOURCES list, and headers to the HEADERS list. Note
+technically you only need to add source files, but omitting headers confuses
+project generation and distribution / packaging targets.
+
+For target conditional files, you can append to the SOURCES or HEADERS lists
+inside an if() test, for example:
+
+    if(APPLE)
+    	list(APPEND SOURCES extraFile1.cxx extraFile2.cxx)
+    endif()
+
+Setting include directories
+
+In any CMakeList.txt, you can do the following:
+
+    include_directories(${PROJECT_SOURCE_DIR}/some/path)
+
+For example, this can be done in particular subdirectory, or at the project
+root, or an intermediate level.
+
+Setting target specific compile flags, includes or defines
+
+Use set_target_property(), for example
+
+    set_target_property(fgfs PROPERTIES
+            COMPILE_DEFINITIONS FOO BAR=1)
+            
+You can set a property on an individual source file:
+
+    set_property(SOURCE myfile.cxx PROPERTY COMPILE_FLAGS "-Wno-unsigned-compare")
+            
+Detecting Features / Libraries
+
+For most standard libraries (Gtk, wxWidget, Python, GDAL, Qt, libXml, Boost),
+cmake provides a standard helper. Check your cmake 'modules' directory to see
+the extensive list. 
+
+In the root CMakeLists file, use a statement like:
+
+    find_package(OpenGL REQUIRED)
+    
+Each package helper sets various variables such aaa_FOUND, aaa_INCLUDE_DIR,
+and aaa_LIBRARY. Depending on the complexity of the package, these variables
+might have different names (eg, OPENSCENEGRAPH_LIBRARIES).
+
+If there's no standard helper for a library you need, find a similar one, copy
+it to CMakeModules/FindABC.cmake, and modify the code to fit. Generally this
+is pretty straightforward.
+
+Note libraries support by pkg-config can be handled directly, with no need
+to create a custom FindABC helper.
+            
+Adding a new executable target
+
+    add_executable(myexecutable ${SOURCES} ${HEADERS})
+    target_link_libraries(myexecutable .... libraries ... )
+    install(TARGETS myexecutable RUNTIME DESTINATION bin)
+    
+(If the executable should not be installed, omit the final line above)
+
+If you add an additional line
+
+    add_test(testname ${EXECUTABLE_OUTPUT_PATH}/myexecutable)
+    
+Then running 'make test' will run your executable as a unit test. The
+executable should return either a success or failure result code.