Add text dealing license around contribution to COPYING

COPYING

@@ -20,3 +20,22 @@ NONINFRINGEMENT. IN NO EVENT SHALL THE 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.
+
+[The text below was composed based on 1.2. License section of
+curl/libcurl project.]
+
+When contributing with code, you agree to put your changes and new
+code under the same license nghttp2 is already using unless stated and
+agreed otherwise.
+
+When changing existing source code, you do not alter the copyright of
+the original file(s).  The copyright will still be owned by the
+original creator(s) or those who have been assigned copyright by the
+original author(s).
+
+By submitting a patch to the nghttp2 project, you are assumed to have
+the right to the code and to be allowed by your employer or whatever
+to hand over that patch/code to us.  We will credit you for your
+changes as far as possible, to give credit but also to keep a trace
+back to who made what changes.  Please always provide us with your
+full real name when contributing!