README: update.

Especially important since it's so prominent on github.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

README

-tools:
-	This is currently a bootstrap junkyard for ccan tools.
+The C Code Archive Network: http://ccodearchive.net
 
-	It is expected that some of this code, being generally useful, will be
-	shuffled out to their own modules over time.
-
-ccan:
-	The beginnings of a ccan repository.
+You can find a set of helper utilities under tools/ and the modules
+under ccan/.  The recommended way to add ccan modules to your project
+is to create a ccan/ directory and add one module per subdirectory
+(see tools/create-ccan-tree).