Update space_requirements.md with recovery steps

src/commands/backup/space_requirements.md

@@ -8,3 +8,22 @@ Should you run out of space during the middle of a backup, there will be some
 additional data in the repository, but the snapshot will never be created as it
 would only be written at the very (successful) end of the backup operation.
 Previous snapshots will still be there and will still work.
+
+# Recovering from out of space
+
+It can be attempted to recover from such situation by running `rustic prune` with
+the following flags:
+
+      --instant-delete
+          Delete files immediately instead of marking them. This also removes all files already marked for deletion.
+
+          # Warning
+
+          * Only use if you are sure the repository is not accessed by parallel processes!
+
+      --early-delete-index
+          Delete index files early if instant-delete is chosen. This allows to run prune if there is few or no space left.
+
+          # Warning
+
+          * If prune aborts, this can lead to a (partly) missing index which must be repaired!