Add upgrade instructions

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+# Upgrade instructions
+This document describes how to upgrade the application and perform eventual database schema upgrades from an older release to a newer release.
+
+For example, if you are coming from `v2.1.2`, you need to perform all database schema upgrades *after* `v2.1.2` (in sequential order) till the release to which you want to upgrade.
+
+## Release `v2.4.1` (coming from `v2.4`)
+For upgrading to this release, a manual database schema upgrade is required. Please execute the following SQL commands inside your blogs database.
+
+~~~sql
+ALTER TABLE `page` MODIFY `argv` VARCHAR(250);
+ALTER TABLE `post` MODIFY `argv` VARCHAR(250);
+ALTER TABLE `user` MODIFY `argv` VARCHAR(250);
+~~~
+
+## Release `v2.4` (coming from `v2.3` or `v2.3.1`)
+For upgrading to this release, a manual database schema upgrade is required. Please execute the following SQL commands inside your blogs database.
+
+**NOTICE:**  
+This upgrade requires MariaDB `>= 10.0.5` or MySQL `>= 5.6`!
+
+~~~sql
+ALTER TABLE `post` ENGINE=InnoDB;
+ALTER TABLE `post` DROP INDEX `body`;
+ALTER TABLE `page` ADD FULLTEXT KEY `search` (`name`, `body`);
+ALTER TABLE `post` ADD FULLTEXT KEY `search` (`name`, `body`);
+ALTER TABLE `post` ADD CONSTRAINT `post_user` FOREIGN KEY (`user`)
+	REFERENCES `user` (`id`) ON DELETE CASCADE ON UPDATE CASCADE;
+~~~
+
+## Release `v2.3` (coming from `v2.2` or `v2.2.1`)
+For upgrading to this release, you need to replace the following strings inside your *customized* theme templates (if you have any).
+
+Search for:  `$Language->template`
+Replace with: `$Language->text`
+
+## Release `v2.2` (coming from `v2.1.2`)
+For upgrading to this release, you need to replace the following strings inside your *customized* theme templates (if you have any).
+
+Search for:  `['ID']`
+Replace with: `['ATTR']['ID']`
+
+## Release `v2.1.2` (coming from `v2.0`, `v2.1` or `v2.1.1`)
+For upgrading to this release, a manual database schema upgrade is required. Please execute the following SQL commands inside your blogs database.
+
+~~~sql
+ALTER TABLE `page` ADD UNIQUE KEY `time_insert` (`time_insert`);
+ALTER TABLE `post` ADD UNIQUE KEY `time_insert` (`time_insert`);
+ALTER TABLE `user` ADD UNIQUE KEY `time_insert` (`time_insert`);
+~~~
+
+## Release `v2.0` (coming from `v1.*`)
+For upgrading to this release, a manual database schema upgrade is required. Please execute the following SQL commands inside your blogs database.
+
+~~~sql
+ALTER TABLE `page` ADD `argv` VARCHAR(100) NULL DEFAULT NULL AFTER `body`;
+ALTER TABLE `post` ADD `argv` VARCHAR(100) NULL DEFAULT NULL AFTER `body`;
+ALTER TABLE `user` ADD `argv` VARCHAR(100) NULL DEFAULT NULL AFTER `body`;
+~~~