adapted PULL_REQUEST_TEMPLATE to ask contributors for version adaption

Author: da-h

PULL_REQUEST_TEMPLATE.md

@@ -27,6 +27,7 @@
 ## Checklist:
 <!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
 <!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
+- [ ] I adapted the version number under `py/visdom/VERSION` according to [Semantic Versioning](https://semver.org/)
 - [ ] My code follows the code style of this project.
 - [ ] My change requires a change to the documentation.
 - [ ] I have updated the documentation accordingly.