Added documentation for the Java API. (spark-jobserver#862)

* Added documentation for the Java API.

* Update javaapi.md

Author: hntd187

doc/javaapi.md

@@ -0,0 +1,41 @@
+##SJS Java Api
+
+The Java API in the Spark Job Server is designed to be a more Java friendly API for users to run Spark Jobs
+inside of SJS.
+
+There are a few differences between this API and the standard one.
+
+- The API is entirely Java based
+- Type safety is done via generics instead of type aliases
+- Requires it's own set of Context factories specific to Java (very important)
+
+###Basic Example
+
+```java
+public class JavaHelloWorldJob implements JSparkJob<String> {
+
+    public String run(JavaSparkContext sc, JobEnvironment runtime, Config data) {
+        return "Hi!";
+    }
+
+    public Config verify(JavaSparkContext sc, JobEnvironment runtime, Config config) {
+        return ConfigFactory.empty();
+    }
+}
+```
+[Full File Here](https://github.com/spark-jobserver/spark-jobserver/blob/master/job-server-tests/src/main/java/spark/jobserver/JavaHelloWorldJob.java)
+
+This example does nothing Spark based, but gives an idea of the difference in structure. In order to run a job
+written like this, the user **MUST** use the `JavaSparkContextFactory`. This context is configured the same as
+any of the Context factories in the Scala API. In addition, there are context types for all Java related jobs.
+In the Java API, there are both context factories and interfaces that must be used for each kind of job.
+For instance the JSparkJob uses the JavaSparkContextFactory.
+
+###Additional Context and Job Types
+
+- `JavaSqlContextFactory` for SQLContext Jobs with `JSqlJob`
+- `JavaStreamingContextFactory` for StreamingContext jobs with `JStreamingJob`
+- `JavaHiveContextFactory` for HiveContext jobs with `JHiveJob`
+
+**NOTE: If you do not use the correct context, the jobs will probably not run correctly, it is imperative you configure
+jobs correctly.**