Merge pull request #519 from dnobori/180426_pullrequest_license

Add a sentence: "preliminary declaration for future change to non-GPL licenses" on pull requests.

.github/PULL_REQUEST_TEMPLATE.md

@@ -12,3 +12,8 @@ You have two options which are described on the above policy.
 Could you please choose either option 1 or 2, and specify it clearly on the reply?
 
 -
+
+PRELIMINARY DECLARATION FOR FUTURE SWITCH TO A NON-GPL LICENSE
+
+I hereby agree in advance that my work will be licensed automatically under the Apache License or a similar BSD/MIT-like open-source license in case the SoftEther VPN Project adopts such a license in future.
+