Add section 'Building and Running' to readme

I thought it might be good to give potential early users some hints on how to quick start the application should they chose to check out the repository.

README.md

@@ -62,6 +62,29 @@ to build its UI. The Intellij IDEA plugin
 provides static previews of
 composable functions without parameters which are annotated with `@Preview`.
 
+### Building and Running
+
+In order to build and run the application from the command line, execute this:
+
+    ./gradlew run
+
+You can specify arguments to the app using the `--args` option of the
+Gradle task. For example to show the usage info:
+
+    ./gradlew run --args="--help"
+
+To specify a different data directory and enable the debug output:
+
+    ./gradlew run --args="--debug --data-dir=/tmp/briar-tmp"
+
+In case you want to build a self-contained Debian package, note that you need at least JDK 15 and that you might need to install `binutils` and `fakeroot` before running the respective task:
+
+    ./gradlew packageDeb
+
+If you want to use the JDKs downloaded by Intellij IDEA, you can for example specify the path like this:
+
+    ./gradlew packageDeb -Dorg.gradle.java.home=/home/briar/.jdks/azul-15.0.4
+
 ## Translations
 
 See [TRANSLATION.md](./TRANSLATION.md) for more information.