PR template: add "docs update"

0xc0170 · 0xc0170 · commit aac58a7e21f9 · 2018-09-25T09:30:58.000+01:00
As we know have documentation in the repository, this type should define changes
only to documentation.
diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
@@ -20,5 +20,6 @@
     [ ] Refactor
     [ ] Target update
     [ ] Functionality change
+    [ ] Docs update
     [ ] Breaking change
 
