kbuild: document environment variables

Add kbuild.txt to Documentation/kbuild
More stuff can be added later - at least we have
som of the varous environment variables documented now.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Documentation/kbuild/00-INDEX b/Documentation/kbuild/00-INDEX
index 54a118a..e8d2b6d 100644
--- a/Documentation/kbuild/00-INDEX
+++ b/Documentation/kbuild/00-INDEX
@@ -1,10 +1,12 @@
 00-INDEX
-    	- this file: info on the kernel build process
+	- this file: info on the kernel build process
+kbuild.txt
+	- developer information on kbuild
+kconfig.txt
+	- usage help for make *config
 kconfig-language.txt
 	- specification of Config Language, the language in Kconfig files
 makefiles.txt
 	- developer information for linux kernel makefiles
-kconfig.txt
-	- usage help for make *config
 modules.txt
 	- how to build modules and to install them