Mention [ci skip] in the pull request template.

[ci skip]

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,5 +3,6 @@ Please include the following checklist in your PR:
 * [ ] Patches conform to the [coding conventions](https://github.com/haskell/cabal/#conventions).
 * [ ] Any changes that could be relevant to users have been recorded in the changelog.
 * [ ] The documentation has been updated, if necessary.
+* [ ] If the change is docs-only, `[ci skip]` is used to avoid triggering the build bots.
 
 Please also shortly describe how you tested your change. Bonus points for added tests!