added vector mutators

docs/source/JavaUsage.md

@@ -177,3 +177,48 @@ There currently is no support for parsing text (Schema's and JSON) directly
 from Java or C#, though you could use the C++ parser through native call
 interfaces available to each language. Please see the
 C++ documentation for more on text parsing.
+
+### Mutating FlatBuffers
+
+As you saw above, typically once you have created a FlatBuffer, it is
+read-only from that moment on. There are however cases where you have just
+received a FlatBuffer, and you'd like to modify something about it before
+sending it on to another recipient. With the above functionality, you'd have
+to generate an entirely new FlatBuffer, while tracking what you modify in your
+own data structures. This is inconvenient.
+
+For this reason FlatBuffers can also be mutated in-place. While this is great
+for making small fixes to an existing buffer, you generally want to create
+buffers from scratch whenever possible, since it is much more efficient and
+the API is much more general purpose.
+
+To get non-const accessors, invoke `flatc` with `--gen-mutable`.
+
+You now can:
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.java}
+    Monster monster = Monster.getRootAsMonster(bb);
+    monster.mutateHp(10);            // Set table field.
+    monster.pos().mutateZ(4);        // Set struct field.
+    monster.mutateInventory(0, 1);   // Set vector element.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+We use the somewhat verbose term `mutate` instead of `set` to indicate that
+this is a special use case, not to be confused with the default way of
+constructing FlatBuffer data.
+
+After the above mutations, you can send on the FlatBuffer to a new recipient
+without any further work!
+
+Note that any `mutate` functions on tables return a boolean, which is false
+if the field we're trying to set isn't present in the buffer. Fields are not
+present if they weren't set, or even if they happen to be equal to the
+default value. For example, in the creation code above we set the `mana` field
+to `150`, which is the default value, so it was never stored in the buffer.
+Trying to call mutateMana() on such data will return false, and the value won't
+actually be modified!
+
+One way to solve this is to call `forceDefaults()` on a
+`FlatBufferBuilder` to force all fields you set to actually be written. This
+of course increases the size of the buffer somewhat, but this may be
+acceptable for a mutable buffer.