Added pull request template

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

@@ -0,0 +1,17 @@
+Hi! Thanks for contributing to Nextflow project.
+
+When submitting a Pull Request please make sure to not include
+in the changeset any modification in these files:
+
+* `nextflow`
+* `docs/conf.py`
+* `modules/nextflow/src/main/groovy/nextflow/Const.groovy`
+
+Also, please sign-off the DCO [1] to certify you are the author of the contribution
+and you adhere to Nextflow open source license [2] adding a `Signed-off-by` line to
+the contribution commit message. For more details check [3].
+
+1. https://developercertificate.org/
+2. https://github.com/nextflow-io/nextflow/blob/master/COPYING
+3. https://github.com/apps/dco
+