Add upgrade notes

docs/simplesamlphp-upgrade-notes-1.18.md

@@ -4,4 +4,11 @@ Upgrade notes for SimpleSAMLphp 1.18
 The minimum PHP version required is now PHP 5.6.
 
 The use of the PHP Memcache-extension was deprecated in favour of the Memcached-extension.
+In order to keep using Memcache-functionality you have to move to the PHP Memchached-extension,
+  which is available from PECL; see https://pecl.php.net/package/memcached
 
+  There are a few options here:
+   - Depending on your distribution, the package may just be available to install
+   - You could use the package from the REMI-repository if you're on RHEL; https://rpms.remirepo.net/
+   - Download the source from https://pecl.php.net/package/memcached and compile the source as a PHP-extension manually;
+     https://www.php.net/manual/en/install.pecl.phpize.php