Always provide "--period" in "--format json" examples.

README

@@ -119,18 +119,24 @@ A mix of both above examples. Order matters !::
 
 Saving the result of an analysis for faster reuse::
 
-  apachedex --default foo --format json --out save_state.json access.log
+  apachedex --default foo --format json --out save_state.json --period day
+  access.log
+
+Although not required, it is strongly advised to provide `--period` argument,
+as mixing states saved with different periods (fixed or auto-detected from
+data) give hard-to-read results and can cause problems if loaded data gets
+converted to a larger period.
 
 Continuing a saved analysis, updating collected data::
 
   apachedex --default foo --format json --state-file save_state.json
-  --out save_state.json access.2.log
+  --out save_state.json --period day access.2.log
 
 Generating HTML output from two state files, aggregating their content
 without parsing more logs::
 
   apachedex --default foo --state-file save_state.json save_state.2.json
-  --out index.html
+  --period day --out index.html
 
 Notes
 =====