[mapdb] Add some documentation (#13964)

This is based on the original OH1 documentation.

Signed-off-by: Wouter Born <github@maindrain.net>

diff --git a/bundles/org.openhab.persistence.mapdb/README.md b/bundles/org.openhab.persistence.mapdb/README.md
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+# MapDB Persistence
+
+The [MapDB](https://mapdb.org/) persistence service is based on simple key-value store that only saves the last value.
+MapDB is useful for restoring items that have the `restoreOnStartup` strategy because other persistence options have some drawbacks if only the last value is needed on restarts.
+
+Some disadvantages of other persistence services compared to MapDB are that they:
+
+* grow in time
+* require complex installs (`influxdb`, `jdbc`, `jpa`)
+* `rrd4j` cannot store all item types (only numeric types)
+
+It is only possible to query the last value and not other historic values because the MapDB persistence service can only store one value per item.