Add Checkstyle with just one rule enabled.

Author: Quintin Willison

README.md

@@ -487,6 +487,16 @@ The Android-specific library AAR is built with:
 
 (The `ANDROID_HOME` environment variable must be set appropriately.)
 
+## Code Standard
+
+We are introducing [Checkstyle](https://checkstyle.org/) to enforce code style and spot for transgressions and illogical constructs.
+The Gradle build has been configured to run these on `java:assembleRelease`.
+It does not run for the Android build yet.
+
+You can run just the Checkstyle rules on their own using:
+
+    ./gradlew checkstyleMain
+
 ## Tests
 
 A gradle wrapper is included so these tasks can run without any prior installation of gradle. The Linux/OSX form of the commands, given below, is:

config/checkstyle/checkstyle.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+		"-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+		"https://checkstyle.org/dtds/configuration_1_3.dtd">
+
+<module name="Checker">
+	<property name="charset" value="UTF-8"/>
+
+	<module name="TreeWalker">
+		<module name="RedundantModifier"/>
+	</module>
+</module>

java/build.gradle

@@ -6,6 +6,7 @@ buildscript {
 
 plugins {
     id 'de.fuerstenau.buildconfig' version '1.1.8'
+	id 'checkstyle'
 }
 
 apply plugin: 'java'
@@ -64,6 +65,7 @@ task fullJar(type: Jar) {
 }
 
 assemble.dependsOn fullJar
+assembleRelease.dependsOn checkstyleMain
 
 configurations {
     fullConfiguration