add more documentation

[[_TOC_]]

## Overview

This is the code to create a Debian package of Google's [Cloud SQL Proxy

program](https://cloud.google.com/sql/docs/mysql/sql-proxy). The package

contains the executable as well as a systemd service. There is a also a

wrapper script `cloudsql-proxy.sh` that is used by the service. This

wrapper script expects to find an options at

`/etc/cloudsql-proxy/options`. This options file contains lines that will

be passed to the `cloudsql-proxy` executable when the systemd service

starts.

Here is an example:

```

# /etc/cloudsql-proxy/options

dir=/run/cloudsql

credential_file=/etc/cloudsql-proxy/cloudsql.json

instances=uit-acs-linux:us-west1:acs-linux-mysql-1=tcp:3306

```

The above options file cause the `ckoudsql-proxy` service to run the following:

```

/usr/bin/cloudsql-proxy -dir=/run/cloudsql -credential_file=/etc/cloudsql-proxy/cloudsql.json -instances=uit-acs-linux:us-west1:acs-linux-mysql-1=tcp:3306

```

## How to build the Debian package

1. Download the latest cloud_sql_proxy executable (64-bit Linux):

        wget https://dl.google.com/cloudsql/cloud_sql_proxy.linux.amd64 -O cloud_sql_proxy