Adds CMakeLists.txt for cpp version

It is good to have CMake compatibility. Demos can be build without
it, but it will improve usability because with CMakeFiles.txt you
just have to specify library path and link library, e.g.
```
add_subdirecory(libs/QR-Code-generator/cpp)
...
target_link_libraries(binary qrcodegen)
```

cpp/CMakeLists.txt

@@ -0,0 +1,7 @@
+project(libqrcodegen)
+
+add_library(qrcodegen STATIC 
+    BitBuffer.cpp QrSegment.cpp QrCode.cpp)
+
+target_include_directories(qrcodegen PUBLIC 
+    ${CMAKE_CURRENT_SOURCE_DIR})