Update usage instructions and separate docs from README

doc/installation.md

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+# Installation
+
+## Installation using Phar
+
+This is the preferred installation method for usage on a production system.
+
+### Requirements
+
+* PHP 5.5+
+
+### Instructions
+
+```bash
+# Go to https://github.com/kelunik/acme-client/releases/latest
+# Download the latest release archive.
+
+# Make it executable.
+chmod +x acme-client.phar
+
+# Run it.
+./acme-client.phar
+
+# Or install it globally.
+mv ./acme-client.phar /usr/local/bin/acme-client
+acme-client
+```
+
+If you want to update, just replace the old `.phar` with a new one.
+
+All commands require a `--storage` argument when using the Phar. That's the path where your keys and certificates will be stored.
+On Unix you could use something like `--storage /etc/acme`.
+
+You can add a file named `acme-client.yml` next to the `.phar` with the two keys `storage` and `server`.
+These values will be used as default if you don't specify them, but you can still use another server by explicitly adding it as argument.
+
+```yml
+# Sample YAML configuration.
+
+# Storage directory for certificates and keys.
+storage: /etc/acme
+
+# Server to use. Available shortcuts: letsencrypt, letsencrypt:staging
+# You can also use full URLs to the directory resource of an ACME server
+server: letsencrypt
+```
+
+## Installation using Composer
+
+If you plan to actively develop this client, you probably don't want the Phar but install the dependencies using Composer.
+
+### Requirements
+
+* PHP 5.5+
+* [Composer](https://getcomposer.org/)
+
+### Instructions
+
+```bash
+# Clone repository
+git clone https://github.com/kelunik/acme-client && cd acme-client
+
+# Install dependencies
+composer install
+```
+
+You can use `./bin/acme` as script instead of the Phar. Please note, that all data will be stored in `./data` as long as you don't provide the `--storage` argument.