Make it clear that the libcouchbase.{so,dll} dependency have to be visible by running process

People do not understand how the DLL loaded, and try to put it in extensions directory of PHP, which does not help container (like apache2) to find it.

https://forums.couchbase.com/t/how-to-make-couchbase-extension-load-on-php-7-0-on-windows/9166/2