add EXPERIMENTAL.

this extension status is still unstable.
i'm going to focus on release this as PECL extension.

PECL already has Discount extension.
but Upskirt has many advantages (more quickly, simple interface...)

please wait for this.

Author: chobie

Diff for: EXPERIMENTAL

@@ -0,0 +1,5 @@
+this extension is experimental,
+its functions may change their names 
+or move to extension all together 
+so do not rely to much on them 
+you have been warned!