Clarify Licensing

[mstewartgallus]

Some sort of COPYING, or LICENSE file would be nice as well as words in the README. Lumen appears to use the ISC license but I can't manually search through all the files in the code to check if they have different licenses.

[darkestkhan]

Outside of the gl/ directory following files don't have license header (all other outside of gl/ are under ISC license):
src/lumen-binary.ads
src/lumen-events.adb
src/x11/x11.ads
src/win32/lumen-window.adb
src/win32/win32.adb
src/win32/win32.ads
src/win32/win32-gdi32.ads
src/win32/win32-opengl32.ads
src/win32/win32-user32.ads

[darkestkhan]

After some checking it turns out that files from src/win32/* are under Afferro GNU General Public License.