LICENSE: add parapraph about doc/ licenses

closes #52

Author: rofl0r

LICENSE

@@ -19,3 +19,7 @@ CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
+-------------------------------------------------------------------------------
+
+Files contained in the doc/ directory may be licensed under different terms.
+If so, it is noted there in the respective file.