docs(readme): Document using s3 vs filesystem locally

Also add a note about some pre-requisites to running this application locally.

closes cfpb#74

README.md

@@ -10,6 +10,58 @@ If you have an inquiry or suggestion for the user and financial institutions man
 - [Poetry](https://python-poetry.org/) is used as the package management tool. Once installed, just running `poetry install` in the root of the project should install all the dependencies needed by the app.
 - [Docker](https://www.docker.com/) is used for local development where ancillary services will run.
 
+---
+### Pre-requisites
+[SBL Project Repo](https://github.com/cfpb/sbl-project) contains the `docker-compose.yml` to run the ancillary services.
+- Not all services need to run, this module `sbl-filing-api` is part of the docker compose file, which doesn't need to be ran in docker for local development.
+- Issuing `docker compose up -d pg keycloak user-fi` would start the necessary services (regtech-fi-user-management, postgres, and keycloak)
+```bash
+$ cd ~/Projects/sbl-project
+$ docker compose up -d pg keycloak user-fi
+[+] Running 3/3
+ ⠿ Network sbl-project_default       Created     0.2s
+ ⠿ Container sbl-project-pg-1        Started     2.6s
+ ⠿ Container sbl-project-keycloak-1  Started     13.4s
+ ⠿ Container sbl-project-user-fi-1   Started     17.3s
+```
+
+----
+## File Upload
+
+This application allows users to upload small business loan register files.  It can be configured to store them in
+either a local filesystem, or an AWS S3 bucket.
+
+### Local Configuration
+
+#### File System
+By default, this application is configured to upload files into a directory named `upload` in the project root when
+running locally within the [.env.local](src/.env.local) file.  The properties involved are:
+```properties
+FS_UPLOAD_CONFIG__PROTOCOL="file"
+FS_UPLOAD_CONFIG__ROOT="../upload"
+```
+
+#### S3
+If you wish to use an Amazon S3 bucket for testing locally:
+
+1. Create an S3 bucket in AWS.  For the purposes of this README, assume it is named `sbl-filing-api`.
+2. Ensure you have access to an AWS user with IAM permissions to read and write to the bucket.
+3. Ensure you have the [AWS CLI](https://aws.amazon.com/cli/) installed.
+4. Ensure you have ran `aws configure` to configure the AWS CLI with credentials necessary to access the given bucket.
+5. Update your environment with the following environment variables (use the bucket name created above in 
+`FS_UPLOAD_CONFIG__ROOT` if it is different):
+    ```bash
+    export FS_UPLOAD_CONFIG__PROTOCOL="s3"
+    export FS_UPLOAD_CONFIG__ROOT="sbl-filing-api"
+    ```
+   This can be made permanent by updating e.g. your `$HOME/.zshrc` file.
+6. Optionally you may update [.env.local](src/.env.local) instead if you prefer.
+7. Restart the application if it is already running.
+
+**Important:** This will make API calls and store files in S3, which may result in charges depending on usage.
+
+If you choose to update `.env.local`, please do not commit that change.
+
 ----
 ## Open source licensing info
 1. [TERMS](TERMS.md)