Add context to installation instructions

Additional clarification of what Composer is, how to install without it, and the development dependencies.

README.md

@@ -17,29 +17,59 @@ with reCAPTCHA.
 
 ## Installation
 
-Use [Composer](https://getcomposer.org/) to install the library. Either use
-`composer require google/recaptcha "~1.1"` or add the following to your
-`composer.json`:
-```json
-    "require": {
-        "google/recaptcha": "~1.1"
-    }
+### Composer (Recommended)
+
+[Composer](https://getcomposer.org/) is a widely used dependency manager for PHP
+packages. This reCAPTCHA client is available on Packagist as
+[`google/recaptcha`](https://packagist.org/packages/google/recaptcha) and can be
+installed either by running the `composer require` command or adding the library
+to your `composer.json`. To enable Composer for you project, refer to the
+project's [Getting Started](https://getcomposer.org/doc/00-intro.md)
+documentation.
+
+To add this dependency using the command, run the following from within your
+project directory:
+```
+composer require google/recaptcha "~1.1"
 ```
 
-### Manual installation
+Alternatively, add the dependency directly to your `composer.json` file:
+```json
+"require": {
+    "google/recaptcha": "~1.1"
+}
+```
 
-If you wish to install the library manually (i.e. without Composer),
-an autoloader is provided as `src/autoload.php`. You can require it in
-your script instead of the usual `vendor/autoload.php` created by
-Composer. For example,
+### Direct download (no Composer)
 
+If you wish to install the library manually (i.e. without Composer), then you
+can use the links on the main project page to either clone the repo or download
+the ZIP file. For convenience, an autoloader script is provided in
+`src/autoload.php` which you can require into your script instead of Composer's
+`vendor/autoload.php`. For example:
 ```php
 require('/path/to/recaptcha/autoload.php');
 
 $recaptcha = new \ReCaptcha\ReCaptcha($secret);
-// etc.
 ```
 
+The classes in the project are structured according to the
+[PSR-4](http://www.php-fig.org/psr/psr-4/) standard, so you may of course also
+use your own autoloader or require the needed files directly in your code.
+
+### Development install
+
+If you would like to contribute to this project or run the unit tests on within
+your own environment you will need to install the development dependencies, in
+this case that means [PHPUnit](https://phpunit.de/). If you clone the repo and
+run `composer install` from within the repo, this will also grab PHPUnit and all
+its dependencies for you. If you only need the autoloader installed, then you
+can always specify to Composer not to run in development mode, e.g. `composer
+install --no-dev`.
+
+*Note:* These dependencies are only required for development, there's no
+requirement for them to be included in your production code.
+
 ## Usage
 
 First, register keys for your site at https://www.google.com/recaptcha/admin
@@ -57,21 +87,23 @@ if ($resp->isSuccess()) {
 }
 ```
 
-You can see an end-to-end working example in [examples/example-captcha.php](examples/example-captcha.php)
+You can see an end-to-end working example in
+[examples/example-captcha.php](examples/example-captcha.php)
 
 ## Upgrading
 
 ### From 1.0.0
 
-The previous version of this client is still available on the `1.0.0` tag
-[in this repo](https://github.com/google/recaptcha/tree/1.0.0) but it is purely
-for reference and will not receive any updates.
+The previous version of this client is still available on the `1.0.0` tag [in
+this repo](https://github.com/google/recaptcha/tree/1.0.0) but it is purely for
+reference and will not receive any updates.
 
 The major changes in 1.1.0 are:
- * installation now via Composer;
- * class loading also via Composer;
- * classes now namespaced;
- * old method call was `$rc->verifyResponse($remoteIp, $response)`, new call is `$rc->verify($response, $remoteIp)`
+* installation now via Composer;
+* class loading also via Composer;
+* classes now namespaced;
+* old method call was `$rc->verifyResponse($remoteIp, $response)`, new call is
+  `$rc->verify($response, $remoteIp)`
 
 ## Contributing