contributing guide: add 5-month maturity quality standard.

CONTRIBUTING.md

@@ -21,6 +21,7 @@ Please contribute links to packages/projects you have used or are familiar with.
 To be on the list, project repositories should adhere to the following quality standards. 
 (https://goreportcard.com/report/github.com/ **github_user** / **github_repo**):
 
+- have at least 5 months of history since the first commit.
 - have an **open source license**, [see list of allowed licenses](https://opensource.org/licenses/alphabetical);
 - function as documented and expected;
 - be generally useful to the wider community of Go programmers;