docs: add release guide (#18)

* docs: add release guide

* docs: add local build guide

Author: DominicGBauer

docs/LocalBuild.md

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+# PowerSync Swift SDK
+
+## Run against a local kotlin build
+
+* To run using the local kotlin build you need to apply the following change in the `Package.swift` file:
+
+  ```swift
+      dependencies: [
+          .package(url: "https://github.com/powersync-ja/powersync-kotlin.git", exact: "x.y.z"), <-- Comment this
+  //        .package(path: "../powersync-kotlin"), <-- Include this line and put in the path to you powersync-kotlin repo
+  ```
+* To quickly make a local build to apply changes you made in `powersync-kotlin` for local development in the Swift SDK run the gradle task `spmDevBuild` in `PowerSyncKotlin` in the `powersync-kotlin` repo. This will update the files and the changes will be reflected in the Swift SDK.

docs/Release.md

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+# PowerSync Swift SDK
+
+## Releasing
+
+* Confirm every PR you want in the release has been merged into `main`.
+* Update `CHANGELOG.md` with the changes.
+* In GitHub actions on GitHub manually run the `Release PowerSync` action. You will be required to update the version and add release notes.
+* If the release notes are complicated and don't fit on a single line it is easier to rather update those after the release is completed by updating the release notes in the new release.