add documentation in snapshots/README.md

snapshots/README.md

@@ -49,6 +49,38 @@ can be trivially forged by an adversary. This was considered out of scope for
 the initial implementation, but can be added later without changes to the
 ABCI state sync protocol.
 
+## Relationship to pruning
+
+Snapshot settings are optional. However, if set, they have an effect on how pruning is going to be done by
+persisting the heights that are multiples of `state-sync.snapshot-interval` until after the snapshot is complete.
+
+If pruning is enabled (not `pruning = "nothing"`), we avoid pruning heights that are multiples of
+`state-sync.snapshot-interval` in the regular logic determined by the 
+pruning settings and applied after every `Commit()`. This is done to prevent a 
+height from being removed before a snapshot is complete. Therefore, we keep 
+such heights until after a snapshot is done. At this point, the height is sent to 
+the `pruning.Manager` to be pruned according to the pruning settings after the next `Commit()`.
+
+To illustrate, assume that we are currently at height 960 with `pruning-keep-recent = 50`,
+`pruning-interval = 10`, and `state-sync.snapshot-interval = 100`. Let's assume that
+the snapshot that was triggered at height `900` just finishes. Then, we can prune height
+`900` right away (that is, when we call `Commit()` at height 960) because it (`900`) is less than `960 - 50 = 910`.
+
+Let's now assume that all settings stay the same but `pruning-keep-recent = 100`. In that case,
+we cannot prune height `900` which is greater than `960 - 100 = 850`. As a result, height 900 is persisted until
+we can prune it according to the pruning settings.
+
+## Configuration
+
+- `state-sync.snapshot-interval`
+   * the interval at which to take snapshots.
+   * the value of 0 disables snapshots.
+   * if pruning is enabled, it is done after a snapshot is complete for the heights that are multiples of this interval.
+
+- `state-sync.snapshot-keep-recent`:
+   * the number of recent snapshots to keep.
+   * 0 means keep all.
+
 ## Snapshot Metadata
 
 The ABCI Protobuf type for a snapshot is listed below (refer to the ABCI spec
@@ -183,7 +215,9 @@ concurrently.
 During `BaseApp.Commit`, once a state transition has been committed, the height
 is checked against the `state-sync.snapshot-interval` setting. If the committed
 height should be snapshotted, a goroutine `BaseApp.snapshot()` is spawned that
-calls `snapshots.Manager.Create()` to create the snapshot.
+calls `snapshots.Manager.Create()` to create the snapshot. Once a snapshot is
+complete and if pruning is enabled, the snapshot height is pruned away by the manager
+with the call `PruneSnapshotHeight(...)` to the `snapshots.types.Snapshotter`.
 
 `Manager.Create()` will do some basic pre-flight checks, and then start
 generating a snapshot by calling `rootmulti.Store.Snapshot()`. The chunk stream