Updated readme

README.md

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+# Getting started
+Docker isn't required, but it is the quickest way to get started.
+
+Copy over the `.env.docker.sample` to `.env`. From that point, make edits to the
+file to get it working.
+
+### Messaging configuration
+The service vendor used to send the messages is `twilio`. After signing up and being
+provided credentials, update the `TWILIO_*` variables.
+
+### Client-Server interaction
+In order for `catapult` to function properly, credentials need to be provided via
+`SERVICE_USERNAME` and `SERVICE_PASSPHRASE`. The credentials are created from making
+an API call to register a service user in `textsender_auth`. So, you need to have credentials
+set in the `.env` file and use those same credentials to create the account in the
+API call.
+
+### API sources
+The `AUTH_URL` and `API_URL` variables need to be set for the base URL for the
+`textsender_auth` and `textsender_api` respectively.
+
+
+Since this is meant to run along with `textsender_auth` and `textsender_api`, it isn't
+meant to be run by itself. So no need to build or run the container in isolation, but
+that capability is present.