[vm] added more docs

docs/vm_functionality.md

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+The VM provides some utilits needed for DApp development.
+
+## ethereumAddress
+This is a constant representing the ethereum address of the user
+
+## renderMessage
+
+## renderModal
+
+## sendETHTransaction
+
+## sendMessage
+`sendMessage(chat <string>, payload <object>, callback <function>)` will send a message with the given parameters.
+The message payload must have the following properties:
+- `shouldSend <bool>` a message can be send or not. If you don't send it it will only be persisted and displayed for you.
+- `params <object>` a set of parameters.
+- `type <string>` a type of the message e.g. `SEND_MONEY` can be used for filtering later.
+
+## setOpenHandler
+`setOpenHandler(handler <function>)` will set a handler that is called when the user opens the DApp.
+
+## setMessageRenderer
+`setMessageRenderer(renderer <function>)` will set a handler for rendering messages. The handler will be called with object (payload) and a callback.
+The callback must be called with an error AND the JSX.
+
+## newModalUIID
+`newModalUIID(closer <function>, callback <function>)` will create a new modal id which you can use to render a modal.
+The closer will be called once the modal get closed. The callback will be called with an error and the modal id.
+The modal ID can then be used to display a modal.
+
+## db
+The database provides four methods:
+- `put(key <string>, value <string>, callback <function>)`
+- `get(key <string>, callback <function>)`
+- `has(key <string>, callback <function>)`
+- `delete(key <string>, callback <function>)`