rewrite README.me to explain

INSTALL

@@ -1,3 +1,8 @@
-Installation steps should be found on http://www.dugwood.com/clickheat/installation.html
+Install it via Piwik Marketplace.
 
-La description de l'installation devrait se trouver sur http://www.dugwood.fr/clickheat/installation.html
+This plugin installer will make directories:
+
+    yourpiwik/tmp/cache/clickheat/cache
+    yourpiwik/tmp/cache/clickheat/logs.
+
+This plugin uses a different tracker. Please click on the link "JavaScript" and put the special Javascript codes into your website.

README.md

@@ -32,7 +32,11 @@ ClickHeat plugin uses text files to record the coordinate data of each browser i
 
 __Does it withstands high traffics ?__
 
-Unfortunately not. Because this plugin uses text files to record data. Therefore, please use this plugin to experiment or low traffics.
+This plugin uses minimal text to record data and file based logging. And when click.php is called from a special Javascript for cgi, just append text on end of the each file. And when you analyze the click data and make a heatmap, plugin will create cached heatmap as png image file. 
+
+Therefore, we expect the plugin light works, but we don't know what load it has under Piwik 2.x. So we are very glad, when you inform us about your situation. 
+
+Please see the link [Performance and optimization](http://www.labsmedia.com/clickheat/156894.html) about system resources. If you want performance, you need to avoid to use a cgi, that is possible. It method is explained on the link. 
 
 __New click data were added, but not updated heatmap. Why ?__
 

plugin.json

@@ -5,7 +5,7 @@
     "theme": false,
     "keywords": ["clickheat", "heatmap", "dugwood", "piwikjapan", "yamachan"],
     "require": {
-        "piwik": ">=2.12.1"
+        "piwik": ">=2.11.1"
     },
     "license": "GPLv3 or later",
     "authors": [