adding notice about additional licenses used in 3rd party code

Author: pthiess

src/license.txt LICENSE

@@ -17,3 +17,7 @@ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 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.
+
+Please note that some portions of this project are written by third parties
+under different license terms. Your use of those portions are governed by
+the license terms contained in the corresponding files.