Add note about different licenses to LICENSE file

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 subdirectories of the CodeMirror distribution
+include their own LICENSE files, and are released under different
+licences.