add a template for pull requests

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,5 @@
+# Checklist for Pull Requests
+
+- [ ] Have you signed the [Contributor License Agreement](https://sftpgo.com/cla.html)?
+
+---