GH-50: include full text of GPL; mention in README

Author: sunaku

README.md

@@ -169,3 +169,9 @@ 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.
+
+(the GPL license)
+
+A small portion of the dasht-query-exec(1) script was derived from [Zeal](
+https://zealdocs.org ), which is licensed under the GNU General Public
+License.  Refer to the `bin/dasht-query-exec` file for more information.