Add CmakeList files to use as external library with Cmake

[gamedolphin]

Now you can add ImGui as a cmake library dependency.

include_directories("${PROJECT_SOURCE_DIR}/src/ImGui")
add_subdirectory ("src/ImGui")
set (EXTRA_LIBS ${EXTRA_LIBS} ImGui)

target_link_libraries(ProjectName ${EXTRA_LIBS})

[ocornut]

Can't take this sorry, don't want a cmakelist file visible by users.
Also see #255

However your code being so small I may include it as a comment somewhere in the documentation.

[gamedolphin]

I understand. 👍 Thanks for this library, its really cool.