Remove firmatabuilder or mark reference as unsupported

Given that firmata-builder (https://github.com/firmata/firmata-builder) hasn't seen updates in seven years and is not fully compatible with ConfigurableFirmata 3.x, it would be helpful to remove or update the reference to the tool in the README.

It is not obvious that this method for configuring Firmata is no longer compatible with the latest version. I ran afoul of this when I tried to use firmatabuilder to create a custom install. Given that recommended method for setting up ConfigurableFirmata is to edit the example (https://github.com/firmata/firmata-builder/issues/24, https://github.com/firmata/firmata-builder/issues/25, and https://github.com/firmata/firmata-builder/issues/26) it seems like keeping folks from stepping into this trap is simple enough.