Added option to install the php calendar extension in php-fpm (#1672)

* Added option to install the php calendar extension in php-fpm
* Added section about installing php calendar extension in php-fpm

env-example

@@ -148,6 +148,7 @@ PHP_FPM_INSTALL_LDAP=false
 PHP_FPM_INSTALL_SWOOLE=false
 PHP_FPM_INSTALL_PG_CLIENT=false
 PHP_FPM_INSTALL_PCNTL=false
+PHP_FPM_INSTALL_CALENDAR=false
 
 ### PHP_WORKER ############################################