PGPRO-1457: note about DELTA mode in README

Author: gsmolk

README.md

@@ -27,6 +27,7 @@ Using `pg_probackup`, you can take full or incremental backups:
 * `Full` backups contain all the data files required to restore the database cluster from scratch.
 * `Incremental` backups only store the data that has changed since the previous backup. It allows to decrease the backup size and speed up backup operations. `pg_probackup` supports the following modes of incremental backups:
   * `PAGE` backup. In this mode, `pg_probackup` scans all WAL files in the archive from the moment the previous full or incremental backup was taken. Newly created backups contain only the pages that were mentioned in WAL records. This requires all the WAL files since the previous backup to be present in the WAL archive. If the size of these files is comparable to the total size of the database cluster files, speedup is smaller, but the backup still takes less space.
+  * `DELTA` backup. In this mode, `pg_probackup` read all data files in PGDATA directory and only those pages, that where changed since previous backup, are copied. Continuous archiving is not necessary for it to operate. Also this mode impose I/O stress equal to `Full` backup and takes the same amount of time.
   * `PTRACK` backup. In this mode, PostgreSQL tracks page changes on the fly. Continuous archiving is not necessary for it to operate. Each time a relation page is updated, this page is marked in a special `PTRACK` bitmap for this relation. As one page requires just one bit in the `PTRACK` fork, such bitmaps are quite small. Tracking implies some minor overhead on the database server operation, but speeds up incremental backups significantly.
 
 Regardless of the chosen backup type, all backups taken with `pg_probackup` support the following archiving strategies: