docs: Enhance Gradle build configuration including how to use tags

issue #119

src/asciidoc/gradle.adoc

@@ -27,4 +27,25 @@ A typical use case is to run the tests and to always produce the aggregate repor
 gradle test aggregate
 ----
 
-This will run the tests and generate an aggregate report in the `target/site/thucydides` directory.
+This will run the tests and generate an aggregate report in the `target/site/thucydides` directory.
+
+Alternatively, configure Gradle as below by first removing the `gradle.startParameter.continueOnFailure = true` line.
+
+Then add the following lines. As per the comments, this will configure the test task so that it will clear the reports first, 
+always execute the Serenity tests, won't stop the build if a test fails, and will automatically run the aggregate task.
+
+This means you only need to execute `gradle test` to execute the tests and don't have to worry about previous runs cluttering up the results of this test execution.
+
+[source,groovy]
+----
+test {
+    // Ensure reports from a previous run are cleared up
+    dependsOn clearReports
+    // Ensure integration tests are always run. Will never report as 'UP-TO-DATE'
+    outputs.upToDateWhen {false}
+    // ensure Gradle ignores failing tests so that the Serenity Aggregation step executes
+    ignoreFailures = true
+    // Ensure Serenity aggregation is run after, and if, integration tests are run
+    finalizedBy aggregate
+}
+----

src/asciidoc/tags.adoc

@@ -137,6 +137,7 @@ You can filter tests by tag while running Serenity.
 With JUnit tests, this can be achieved by providing a single tag or a comma separated list of tags from command line.
 If provided, only classes and/or methods with tags in this list will be executed.
 
+===== Maven Build
 [source,xml]
 ----
 mvn clean verify -Dtags="release:sprint-2"
@@ -149,6 +150,30 @@ or
 mvn clean verify -Dtags="feature:Reporting, release:sprint-2"
 ----
 
+===== Gradle Build
+In order to filter with Gradle, the following needs to be added to the configuration of the test task. If it is not present, the value of 'tags' does not get passed to the SerenityRunner, and the tags filter is not applied.
+
+[source,groovy]
+----
+test {
+    systemProperty 'tags', System.properties['tags']
+}
+----
+
+Once this configuration is in place, one can filter in a similar manner to Maven. 
+
+[source,xml]
+----
+gradle test verify -Dtags="release:sprint-2"
+----
+
+or
+
+[source,xml]
+----
+gradle test -Dtags="feature:Reporting, release:sprint-2"
+----
+
 ==== Cucumber
 
 With Cucumber framework, you need to use the cucumber.options system property for tests filtering.