Add more configuration info to README

Author: fhunleth

README.md

@@ -4,6 +4,40 @@ Interact with simulated I2C devices
 
 NOTE: This project is an active development. Expect big API changes as we use it.
 
+## Using
+
+CircuitsSim requires Elixir Circuits 2.0 libraries. Currently, this requires the
+following in your `mix.exs`:
+
+```elixir
+  defp deps() do
+    [
+      {:circuits_i2c, "~> 2.0"},
+      {:circuits_spi, "~> 2.0.0-pre.0"},
+      {:circuits_gpio, "~> 2.0.0-pre.0"}
+    ]
+  end
+```
+
+You may need to add `override: true` since not many libraries have been updated
+to allow Circuits 2.0 versions. The update is generally compatible, so this is
+safe.
+
+CircuitsSim works by providing an alternative backend for interacting with
+hardware. This is setup via `config/config.exs`. In your project, you may only
+want the simulated backends for `MIX_ENV=test` configurations, so you'll want to
+add the following appropriately. For trying it out, it's fine to start with
+adding this to `config.exs`:
+
+```elixir
+config :circuits_i2c, default_backend: CircuitsSim.I2C.Backend
+config :circuits_spi, default_backend: CircuitsSim.SPI.Backend
+config :circuits_gpio, default_backend: CircuitsSim.GPIO.Backend
+```
+
+Note that you don't need to replace all backends. If you're only using I2C,
+there's no need to bring in SPI and GPIO support.
+
 ## Demo
 
 CircuitsSim takes a configuration for how to set up the simulated I2C buses and