Add javadoc

build.gradle

@@ -19,6 +19,20 @@ sourceCompatibility = 1.8
 
 def grpc_java_version = '1.14.0'
 
+tasks.withType(JavaCompile) {
+    options.encoding = "UTF-8"
+}
+
+javadoc {
+    source = sourceSets.main.allJava
+    classpath = configurations.compile
+    options {
+        encoding 'UTF-8'
+        charSet 'UTF-8'
+        title "BDChain Java SDK API"
+    }
+}
+
 protobuf {
     protoc {
         artifact = "com.google.protobuf:protoc:3.6.0"