Document --enable-redis-igbinary configure flag

Author: nicolasff

README.markdown

@@ -11,10 +11,11 @@ Installing/Configuring
 
 <pre>
 phpize
-./configure
+./configure [--enable-redis-igbinary]
 make && make install
 </pre>
 
+If you would like phpredis to serialize your data using the igbinary library, run configure with `--enable-redis-igbinary`.
 `make install` copies `redis.so` to an appropriate location, but you still need to enable the module in the PHP config file. To do so, either edit your php.ini or add a redis.ini file in `/etc/php5/conf.d` with the following contents: `extension=redis.so`.
 
 You can generate a debian package for PHP5, accessible from Apache 2 by running `./mkdeb-apache2.sh` or with `dpkg-buildpackage` or `svn-buildpackage`.