Added a new form to display the GPLv3 license for the project.

Resolves issue #6.

LicenseForm.cs

@@ -0,0 +1,22 @@
+using System.Diagnostics;
+
+namespace PDF_Merge
+{
+    public partial class LicenseForm : Form
+    {
+        public LicenseForm()
+        {
+            InitializeComponent();
+        }
+
+        private void GPLLink_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
+        {
+            Process.Start(new ProcessStartInfo("https://www.gnu.org/licenses/gpl-3.0.en.html") { UseShellExecute = true });
+        }
+
+        private void CloseBtn_Click(object sender, EventArgs e)
+        {
+            Close();
+        }
+    }
+}