Update the README in preparation for php-memcached 3.x release

Author: sodabrew

README.markdown

@@ -1,15 +1,15 @@
 Build Status
 ------------
-[![Build Status](https://travis-ci.org/php-memcached-dev/php-memcached.png?branch=php7)](https://travis-ci.org/php-memcached-dev/php-memcached)
+[![Build Status](https://travis-ci.org/php-memcached-dev/php-memcached.png)](https://travis-ci.org/php-memcached-dev/php-memcached)
 
 Description
 -----------
-This extension uses libmemcached library to provide API for communicating with
-memcached servers.
+This is the [PECL memcached](https://pecl.php.net/package/memcached) extension,
+using the libmemcached library to connect to memcached servers.
 
-memcached is a high-performance, distributed memory object caching system,
-generic in nature, but intended for use in speeding up dynamic web applications
-by alleviating database load.
+[memcached](https://memcached.org) is a high-performance, distributed memory
+object caching system, generic in nature, but intended for use in speeding up
+dynamic web applications by alleviating database load.
 
 Building
 --------
@@ -19,8 +19,19 @@ Building
     $ make
     $ make test
 
-Resources
----------
- * [libmemcached](http://libmemcached.org/libMemcached.html)
- * [memcached](http://memcached.org/)
- * [igbinary](https://github.com/igbinary/igbinary)
+Dependencies
+------------
+
+php-memcached 3.x releases support PHP 7.0 - 7.1, and require libmemcached 1.x or higher, and optionally support igbinary 2.0 or higher.
+
+php-memcached 2.x releases support PHP 5.2 - 5.6, and require libmemcached 0.44 or higher, and optionally support igbinary 1.0 or higher.
+
+[libmemcached](http://libmemcached.org/libMemcached.html) version 1.0.16 or
+higher is recommended for best performance and compatibility with memcached
+servers.
+
+[igbinary](https://github.com/igbinary/igbinary) is a faster and more compact
+binary serializer for PHP data structures. When installing php-memcached from
+source code, the igbinary module must be installed first so that php-memcached
+can access its C header files. Load both modules in your `php.ini` at runtime
+to begin using igbinary.