Add CONTRIBUTING to detail what contributors/maintainers do

Author: ApolloZhu

.github/pull_request_template.md

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+- [ ] I understand that GitHub is only a read-only mirror of the KDE project and will instead open a merge request on [KDE's GitLab](https://invent.kde.org/network/kdeconnect-ios)

CONTRIBUTING.md

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+# Getting Involved with KDE Connect iOS
+
+There are many aspects of contributing to and help maintaining the KDE Connect iOS project.
+
+## Contributors
+
+### Adding Features, Documentation, and Fixing Bugs
+
+Checking and updating [Bugs](https://bugs.kde.org/buglist.cgi?cmdtype=runnamed&list_id=2392382&namedcmd=KDE%20Connect%20iOS)
+and [Phabricator](https://phabricator.kde.org/project/board/159/) tasks (tagged `iOS`),
+as well as documenting about how to use/develop KDE Connect iOS are all valuable contributions
+through either [KDE's GitLab](https://invent.kde.org/network/kdeconnect-ios) or one of the Wiki's:
+
+- [KDE Connect iOS](https://invent.kde.org/network/kdeconnect-ios/-/wikis/home)
+- [Community](https://community.kde.org/KDEConnect)
+- [UserBase](https://userbase.kde.org/KDEConnect)
+
+### Performing Code Review
+
+Testing the application to see if the app works as intended and/or
+to see if the code follows best practices. Here's a checklist that might be helpful:
+[Reviewing Merge Requests](https://invent.kde.org/network/kdeconnect-ios/-/wikis/Reviewing-Merge-Requests)
+
+### Spreading the Words
+
+Many open source beginners are looking for exciting projects to join,
+and you can let them know about KDE Connect and other KDE projects.
+Especially for KDE Connect iOS, the people who can help out the most (iOS developers)
+are likely unfamiliar with KDE community, so going to communities
+such as [Xcoders](https://xcoders.org) and [iOS Dev Happy Hour](https://www.iosdevhappyhour.com),
+regional events like [try! Swift](https://www.tryswift.co)
+or global conferences like [WWDC](http://developer.apple.com/wwdc),
+or posting to online forums like [r/swift](https://www.reddit.com/r/swift/) on Reddit,
+sharing your experiences working on KDE Connect iOS
+or deliver talks about knowledge you learned by working with this project
+can all be great ways to increase liveliness with this project.
+
+## Maintainers
+
+Once you've become a frequent contributor to the KDE Connect iOS project,
+an existing maintainer can recommend you to join KDE's Apple Developer team
+and access App Store Connect, which allows you to
+
+- upload new TestFlight public beta builds and fill in the release notes,
+   + ideally, this should be automated
+- make a new release for everyone to download from App Store
+   + the next beta build needs a new Version number and takes longer to be reviewed by App Store review team),
+- see crash reports and tester feedback to prioritize high-impact and triage obscure bugs, and
+- answering customer reviews from App Store
+   + they'll get you reply as an email!
+   + you can also search online (e.g. Reddit, YouTube) to see what people are complaining about
+     and see how we can change to improve the user experience
+
+Don't know how? Check out [Wiki](https://invent.kde.org/network/kdeconnect-ios/-/wikis/home#app-store-releases-and-beta-testing)
+or documentation from Apple Developer site.