contributing: clarify 3rd-party service symbol requirement

ref https://github.com/Kickball/awesome-selfhosted/pull/739

.github/CONTRIBUTING.md

@@ -2,11 +2,12 @@
 
 Please join us on [chat](https://chat.awesome-selfhosted.com) to clarify any questions.
 
- * The **syntax** for links is ``[Name](http://homepage/) - Short description. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code)) `License` `Language```. All projects should have their **license** and main server-side **language**/platform/requirement listed, and a description at most 250 characters long. The ⚠ character must be inserted after a service's name that relies on a third party service, e.g ``[Name](http://homepage/) ⚠ - Short description.```.
+ * The **syntax** for links is ``[Name](http://homepage/) - Short description. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code)) `License` `Language` ``. All projects should have their **license** and main server-side **language**/platform/requirement listed, and a description at most 250 characters long.
  * To **add an entry**: clone/[download](https://github.com/Kickball/awesome-selfhosted/archive/master.zip) and enter the repository, run `make add` and follow the instructions; or [edit the README.md file](https://github.com/Kickball/awesome-selfhosted/edit/master/README.md) directly. See [Editing files in another user's repository](https://help.github.com/articles/editing-files-in-another-user-s-repository/), [Creating Pull Requests](https://help.github.com/articles/creating-a-pull-request/), [Using Pull Requests](https://help.github.com/articles/using-pull-requests/) for help on sending your patch.
  * A  minimum of 3 entries are required to start a new category, otherwise add to the Misc/Other category.
  * Entries should be ordered alphabetically.
  * Non-[Free](https://en.wikipedia.org/wiki/Free_software) software must be marked `⊘ Proprietary` next to the title, and added to [non-free.md](non-free.md).
+ * The `⚠` character must be inserted for software that relies on a third party service the user does not control, e.g ``[Name](http://homepage/) `⚠` - Short description...``.
  * Check current [issues](https://github.com/Kickball/awesome-selfhosted/issues) and [Pull Requests](https://github.com/Kickball/awesome-selfhosted/pulls), as well as [closed issues/PRs](https://github.com/Kickball/awesome-selfhosted/pulls?utf8=%E2%9C%93&q=is%3Aclosed+).
  * The list of contributors can be updated with `make contrib`. Several checks are available: `make noexternallink; make missinglicenselanguage; make nofullstop` 
  * Software with no development activity for 6-12 months may be removed from the list.