document the embedded API for Java

doc/runtime-api.html

@@ -650,6 +650,26 @@ visit = case fg e2 of
 </pre>
 </p>
 </span>
+<span class="java">
+<p>In order to access the API you first need to generate
+one boilerplate Java class with the compiler:
+<pre class="java">
+$ gf -make -output-format=java App.pgf
+</pre>
+This class will expose all functions in the abstract syntax
+as methods. Now creating new trees is just a matter of writing ordinary Java
+code:
+<pre class="java">
+System.out.println(App.DetCN(quant, cn));
+</pre>
+If the grammar name is too long to write it in front of every function
+name then you can create an instance with a shorter name:
+<pre class="java">
+App a = new App();
+System.out.println(a.DetCN(quant, cn));
+</pre>
+</p>
+</span>
 
 <span class="python">
 <p>