Fill README of version-sync Gradle plugin

gradle-plugins/version-sync/README.md

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+# Version sync
+
+Synchronizes the `version` in multi-module projects, and allows to also generate code with the version inside.
+
+## The 2 things this Gradle plugin does
+
+### 1. Set version to all modules/projects from a single file
+
+By default, the version is retrieved from the first line of the `version.txt` at the root of the project,
+and it set to all the modules/projects (in the `version` property that all Gradle modules/projects have).
+
+It is possible to override the version in a given project if needed.
+
+You can also change the version file like this:
+
+```gradle.kts
+versionSync { versionFile = file("library-version.txt") }
+```
+
+#### Why a version **file**?
+
+The reason we put the version in a plain text file is because it allows it to be used as CI triggers too,
+and any tool can easily read it without having to parse Kotlin or Groovy based Gradle files.
+
+### 2. Embed the version in the code of a project
+
+A Gradle plugin, or anything you make might need to know its own version.
+
+For example, the SqlDelight Gradle plugin needs to know its version for driver dependencies for host/end app projects. 
+
+With the Version sync Gradle plugin, you can do it easily from your `build.gradle.kts` file:
+
+```gradle.kts
+import org.splitties.incubator.gradle.putVersionInCode
+
+// Rest of your build code...
+
+sourceSets { main { kotlin.srcDir("build/gen") } } // Add custom directory for generated code
+
+putVersionInCode(
+ outputDirectory = layout.dir(provider { file("build/gen") }), // Output in the same custom directory
+ writer = VersionFileWriter.Kotlin(
+ fileName = "Version.kt",
+ `package` = "splitties.wff",
+ // Optional `public` parameter (defaults to `false`, generating `internal` visibility).
+ propertyName = "thisProjectVersion",
+ const = true // Defaults to false. Recommended to be left to false if made public.
+ )
+)
+```