Update pull request template to be clearer for coverage service link section
- repo link (github.com, gitlab.com, etc):
- pkg.go.dev:
- goreportcard.com:
-- coverage service link ([codecov](https://codecov.io/), [coveralls](https://coveralls.io/), [gocover](http://gocover.io/) etc.)
-
-Very good coverage
+- coverage service link ([codecov](https://codecov.io/), [coveralls](https://coveralls.io/), [gocover](http://gocover.io/) etc.):
**Note**: _that new categories can be added only when there are 3 packages or more._
