pull request template: add checkbox for awesome-sysadmin and static site generators

Clarifies a rule which was previously manually "enforced" by maintainers
(see closed issues/pull requests)
Prevents duplication of effort/information
Prevents contributors from wasting their time

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,7 +6,8 @@ To ensure your PR is dealt with swiftly please check the following:
         
     ``[Name](http://homepage/) - Short description. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code)) `License` `Language```
 - [ ] Your additions are ordered alphabetically.
-- [ ] Your additions are [free software](https://en.wikipedia.org/wiki/Free_software), or if not they have been added to [non-free](non-free.md).
+- [ ] Your additions are [Free software](https://en.wikipedia.org/wiki/Free_software), or if not they have been added to [non-free](non-free.md).
+- [ ] Your additions are not already listed at [awesome-sysadmin](https://github.com/n1trux/awesome-sysadmin) (IT infrastructure management), [staticgen.com](https://www.staticgen.com/) or [staticsitegenerators.net](https://staticsitegenerators.net/) (static site generators).
 - [ ] Any licenses you have added are in our [list of licenses](https://github.com/Kickball/awesome-selfhosted/blob/master/README.md#list-of-licenses).
 - [ ] You have searched the repository for any relevant issues or PRs.
 - [ ] Any category you are creating has the minimum requirement of 3 items.