Update usage instructions and separate docs from README

doc/usage.md

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+# Usage
+
+**The client stores all data in `./data` if you're using the Composer installation method, otherwise in the directory you configured. Be sure to backup this folder regularly. It contains your account keys, domain keys and certificates.**
+
+Before you can issue certificates, you have to register an account first and read and understand the terms of service of the ACME CA you're using.
+For the Let's Encrypt certificate authority, there's a [subscriber agreement](https://letsencrypt.org/repository/) you have to accept.
+
+By using this client you agree to any agreement and any further updates by continued usage.
+You're responsible to react to updates and stop the automation if you no longer agree with the terms of service.
+
+These usage instructions assume you have installed the client globally as a Phar. If you are using the Phar, but don't have it globally, replace `acme-client` with the location to your Phar.
+
+If you're using the client with Composer, replace `acme-client` with `bin/acme`. You have to specify the server with `-s` / `--server`, because there's currently no config file support for this installation method.
+
+## Register an Account
+
+```
+acme-client setup --email me@example.com
+```
+
+After a successful registration you're able to issue certificates.
+This client assumes you have a HTTP server setup and running.
+You must have a document root setup in order to use this client.
+
+## Issue a Certificate
+
+```
+acme-client issue -d example.com:www.example.com -p /var/www/example.com
+```
+
+You can separate multiple domains (`-d`) with `,`, `:` or `;`. You can separate multiple document roots (`-p`) with your system's path separator:
+ * Colon (`:`) for Unix
+ * Semicolon (`;`) for Windows
+
+If you specify less paths than domains, the last one will be used for the remaining domains.
+
+Please note that Let's Encrypt has rate limits. Currently it's five certificates per domain per seven days. If you combine multiple subdomains in a single certificate, they count as just one certificate. If you just want to test things out, you can use their staging server, which has way higher rate limits by appending `--s letsencrypt:staging`.
+
+## Revoke a Certificate
+
+To revoke a certificate, you need a valid account key, just like for issuance.
+
+```
+acme-client revoke --name example.com
+```
+
+`--name` is the common name of the certificate that you want to revoke.
+
+## Renewing a Certificate
+
+For renewal, there's the `acme-client check` subcommand.
+It exists with a non-zero exit code, if the certificate is going to expire soon.
+Default check time is 30 days, but you can use `--ttl` to customize it.
+
+You may use this as daily cron:
+
+```
+acme-client check --name example.com || acme-client issue ...
+```
+
+You can also use a more advanced script to automatically reload the server as well. For this example we assume you're using Nginx. Something similar should work for Apache.
+
+```bash
+#!/usr/bin/env bash
+
+acme-client check --name example.com --ttl 30
+
+if [ $? -eq 1 ]; then
+        acme-client issue -d example.com:www.example.com -p /var/www
+
+        if [ $? -eq 0 ]; then
+                nginx -t -q
+
+                if [ $? -eq 0 ]; then
+                        nginx -s reload
+                fi
+        fi
+fi
+```