Added a Readme to the backend. (#401)

backend/README.md

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+# Police Data Index Backend
+
+This is the backend for the National Police Data Index project. It is a Python Flask application that serves as the API for the front end.
+
+## Database
+
+The backend uses a PostgreSQL database to store data. The database schema is defined in [database/models](https://github.com/codeforboston/police-data-trust/tree/a743c232b5737b193086264e1364b1475873a884/backend/database/models).
+
+You can access the database using the credentials you stored in the `.env` file. With those credentials, you can connect to the database and interact with it using SQL commands. You can do this by either using the command line or a GUI tool.
+
+### Command Line
+
+To connect to the database using the command line, you can use the `psql` command. You can run the following command to connect to the database:
+```bash
+psql -h localhost -U your_username -d your_database
+```
+
+### GUI Tool
+We recommend using PGAdmin to interact with the database. You can download it [here](https://www.pgadmin.org/download/).
+
+You can connect to the database by creating a new server and entering the credentials you stored in the `.env` file.
+
+## Authentication
+
+Before you can send requests to the API, you must first authenticate. Each API request must include an `Authorization` header with a valid JWT token. To obtain this, we'll be creating a user account and then logging in to get the token. 
+
+### Create a User
+
+To create a user account, send a POST request to the [`/register`](https://github.com/codeforboston/police-data-trust/blob/a743c232b5737b193086264e1364b1475873a884/backend/routes/auth.py#L62) endpoint with the correct payload. 
+
+The response will include the User information as well as a JWT token that you can use to authenticate your API requests.
+
+### Login
+
+To login, send a POST request to the [`/login`](https://github.com/codeforboston/police-data-trust/blob/a743c232b5737b193086264e1364b1475873a884/backend/routes/auth.py#L20) endpoint with the correct payload.
+
+The response will include a JWT token that you can use to authenticate your API requests.
+
+### Using the Token
+
+To authenticate your API requests, include the JWT token in the `Authorization` header. The header should look like this:
+```
+Authorization: Bearer <your_token>
+```