Add performance tips in README.

README

@@ -151,6 +151,25 @@ without parsing more logs::
   apachedex --default foo --state-file save_state.json save_state.2.json
   --period day --out index.html
 
+Performance
+===========
+
+For better performance...
+
+- pipe decompressed files to apachedex instead of having apachedex decompress
+  files itself::
+
+    bzcat access.log.bz2 | apachedex [...] -
+
+- parse log files in parallel processes, saving analysis output and aggregating
+  them in the end::
+
+    for LOG in access*.log; do
+      apachedex "$@" --format json --out "$LOG.json" "$LOG" &
+    done
+    wait
+    apachedex "$@" --out access.html --state-file access.*.json
+
 Notes
 =====