add license title

It's not strictly required, but it's useful metadata, and part of the recommended license template text (see http://choosealicense.com/licenses/isc/ and https://opensource.org/licenses/isc-license)

COPYRIGHT

@@ -1,5 +1,7 @@
 PgBouncer - Lightweight connection pooler for PostgreSQL.
 
+ISC License
+
 Copyright (c) 2007-2009  Marko Kreen, Skype Technologies OÜ
 
 Permission to use, copy, modify, and/or distribute this software for any
@@ -13,4 +15,3 @@ ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
-