README: Document __SSIC__.

diff --git a/README b/README
index c7a39557853c362801192f79aca9566c6ed13d9e..0eface8fbfd264d5e2e4c350939e2a7ae4c635c5 100644 (file)
--- a/README
+++ b/README
@@ -61,6 +61,8 @@ Include Variables
 
 The following variables are set:
 
+  * `__SSIC__` indicates that the Server Side Includes Compiler is being
+    used to process the document's SSI directives.
   * `DATE_GMT` is the current date in UTC.
   * `DATE_LOCAL` is the current date in the local timezone.
   * `DOCUMENT_NAME` is the current filename.