feat: add README

README.md

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+# Digit Scanner
+
+[![ci](https://github.com/Scalamando/digit-scanner/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/Scalamando/digit-scanner/actions/workflows/ci.yml)
+[![docker_size](https://badgen.net/docker/size/scalamando/digit-scanner/latest?icon=docker&label=size)](https://hub.docker.com/repository/docker/scalamando/digit-scanner)
+
+A web app to collect handwritten digits for data-science use cases. Built with Express, PostgreSQL and TypeORM.
+
+## Usage
+
+Make sure to set the required environment variables. Otherwise the app will most likely fail to connect to your PostgreSQL database.
+
+Example:
+
+```bash
+POSTGRES_HOST       = localhost
+POSTGRES_USER       = postgres
+POSTGRES_PASSWORD   = postgres
+POSTGRES_DB         = digits
+POSTGRES_PORT       = 5432
+PORT                = 3000
+
+# For use with CI
+CA_CERT             = xxxx
+```
+
+## Development
+
+Make sure that the `PORT` environment variable is set to your needs. If no value is found, the port defaults to `3000`.
+
+```bash
+# install dependencies
+$ yarn install
+
+# start express app and serve at localhost:$PORT (default: 3000)
+$ yarn start:dev
+```
+
+If you want to use the swagger docs, do the following:
+
+```bash
+# build OpenAPI spec and generate Swagger docs
+$ yarn pre:build
+```
+
+The docs will then be served at `/docs`.
+
+## Deployment
+
+Use the following commands to build the application:
+
+```bash
+# generate docs and build application
+yarn pre:build && yarn build
+
+# start the application
+yarn start
+```
+
+Alternatively, use the [docker image](https://hub.docker.com/repository/docker/scalamando/digit-scanner) or build it yourself with:
+
+```bash
+docker build -t digit-scanner .
+```
+
+Use the .env file or the equivalent in your deployment environment to set the database parameters.
+
+**Important:** If ssl is not disabled the `CA_CERT` key is required. Otherwise the database will reject the connection.
+
+Example:
+
+```bash
+POSTGRES_HOST       = ${ DATABASE_HOST }
+POSTGRES_USER       = ${ DATABASE_USER }
+POSTGRES_PASSWORD   = ${ DATABASE_PASSWORD }
+POSTGRES_DB         = ${ DATABASE_DB }
+POSTGRES_PORT       = ${ DATABASE_PORT }
+CA_CERT             = ${ DATABASE_CERT }
+PORT                = ${ PORT }
+```