kbuild: add documentation

I have missed a place to document how to use kbuild. We already have:

makefiles.txt - which specify mostly the kbuild 'language'
used in the kbuild files.

modules.txt - which deals with modules only.

So I went on and created a file named kbuild.txt. For now it contains only a list of environment variables but we can add more chapters over time now we have a place to do so.

Comments (and updates!) are welcome.

Note: kconfig.txt is a new file - I have not yet pushed it out as I am waiting for Linus to pull the current kbuild bits.

Sam

index 54a118a..e8d2b6d 100644
@@ -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