doc: add pull request template

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- DO NOT DELETE THE TEXT BELOW if you are adding new software to the list. Please make sure relevant boxes are checked [x] -->
+
+Thanks for taking the time to suggest an addition to awesome-selfhosted! 
+
+To ensure your Pull Request is dealt with swiftly, please check the following (check the boxes `[x]`):
+- [ ] Submit one item per pull request. This eases reviewing and speeds up inclusion.
+- [ ] You have searched the repository for any relevant [issues](https://github.com/awesome-selfhosted/awesome-selfhosted-data/issues) or [PRs](https://github.com/awesome-selfhosted/awesome-selfhosted-data/pulls), including closed ones.
+- [ ] Any software you are adding is not already listed at any of [awesome-sysadmin](https://github.com/n1trux/awesome-sysadmin), [staticgen.com](https://www.staticgen.com/), [staticsitegenerators.net](https://staticsitegenerators.net/), [dbdb.io](https://dbdb.io/browse).
+- [ ] The file you are adding is formatted as described in [addition.md](https://github.com/awesome-selfhosted/awesome-selfhosted/blob/master/.github/ISSUE_TEMPLATE/addition.md).
+- [ ] Comments and unused optional fields have been removed.
+- [ ] The file you are adding uses [kebab-case](https://en.wikipedia.org/wiki/Letter_case#Kebab_case) file naming, for example `my-awesome-software.yml`.
+- [ ] Values for `platform` are the main **server-side** requirements for the software. Don't include frameworks or specific dialects.
+- [ ] Any software project you are adding to the list is actively maintained.
+- [ ] Any software project you are adding was first released more than 4 months ago.
+- [ ] Any software project you are adding has working installation instructions.