[Chore] Mention Unreleased section in PR template

Problem: it's not really fixed anywhere, whether should we have an empty
`Unreleased` section in our changelog, or better go without it.

Solution: we agreed with @Gromak that it's better to leave no empty
section (so that it does not appear on Hackage).

So as a clarification, on our PR template I change the point about
changelog (for regular PRs, not as part of release routine) to mention
the creation of `Unreleased` section if it is absent.

Author: Martoon-00

.github/pull_request_template.md

@@ -49,7 +49,7 @@ checkmark indicating that you are sure it is dealt with (be that by irrelevance)
 
 - Record your changes
 
-  - [ ] I added an entry to the [changelog](https://github.com/serokell/universum/tree/master/CHANGES.md) if my changes are visible to the users
+  - [ ] I added an entry to the [changelog](https://github.com/serokell/universum/tree/master/CHANGES.md) (creating the `Unreleased` section if necessary) if my changes are visible to the users
         and
   - [ ] provided a migration guide for breaking changes if possible