Add cache doc in README.

README.md

@@ -148,3 +148,21 @@ Add a dedicated `host` section in `/etc/tomcat10/server.xml`:
 
 The `cache` directory must be readable by Tomcat.
 
+## About page caching
+
+The web pages are put in a HTML cache so they are not build again at each request.
+
+The HTML cache is automatically clear when the data directory is modified. The reload is quick.
+
+However, the organizations and services metrics pages are in cache. Just because the combinatorial is too large!
+
+Example:
+
+- (Summary,Generic,Specific,Web) x (Month, Weeks) x (ALL, last months, year1-year4) = 48 pages by item
+- 100 organizations + 300 services = 400 items
+- 400 items x 48 pages => ~20 000 pages
+- 200kB/page => 4GB in memory
+
+Metrics pages are so numerous than they have to build on demand.
+
+