Create How To Build

How-To-Build.md

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+1. [Install flutter](https://flutter.dev)
+
+2. Clone the repo:
+```
+git clone --recurse-submodules https://gitlab.com/famedly/fluffychat
+cd fluffychat
+```
+
+3. Choose your target platform below and enable support for it.
+
+3.1 If you want, enable Googles Firebase Cloud Messaging:
+
+`git apply ./scripts/enable-android-google-services.patch`
+
+4. Debug with: `flutter run`
+
+### Android
+
+* Install CMake from the SDK Manager
+
+* Install ninja:
+```
+sudo apt install ninja-build
+```
+
+* Build with: `flutter build apk`
+
+### iOS / iPadOS
+
+* Have a Mac with Xcode installed, and set up for Xcode-managed app signing
+* If you want automatic app installation to connected devices, make sure you have Apple Configurator installed, with the Automation Tools (`cfgutil`) enabled
+* Set a few environment variables
+    * FLUFFYCHAT_NEW_TEAM: the Apple Developer team that your certificates should live under
+    * FLUFFYCHAT_NEW_GROUP: the group you want App IDs and such to live under (ie: com.example.fluffychat)
+    * FLUFFYCHAT_INSTALL_IPA: set to `1` if you want the IPA to be deployed to connected devices after building, otherwise unset
+* Run `./scripts/build-ios.sh`
+
+### Web
+
+* Enable web support in Flutter: https://flutter.dev/docs/get-started/web
+
+* Build with:
+```bash
+./scripts/prepare-web.sh # To install libolm
+flutter build web --release
+```
+
+* Optionally configure by serving a `config.json` at the same path as fluffychat.
+  An example can be found at `config.sample.json`. None of these
+  values have to exist, the ones stated here are the default ones. If you e.g. only want
+  to change the default homeserver, then only modify the `default_homeserver` key.
+
+### Desktop (Linux, Windows, macOS)
+
+* Enable Desktop support in Flutter: https://flutter.dev/desktop
+
+* Build with one of these:
+```bash
+flutter build linux --release
+flutter build windows --release
+flutter build macos --release
+```
+
+For encryption support you have to install [libolm](https://gitlab.matrix.org/matrix-org/olm) on your system.