guix: Update conservative space requirements

contrib/guix/README.md

@@ -13,11 +13,9 @@ We achieve bootstrappability by using Guix as a functional package manager.
 
 Conservatively, a x86_64 machine with:
 
-- 4GB of free disk space on the partition that /gnu/store will reside in
-- 24GB of free disk space on the partition that the Bitcoin Core git repository
-  resides in
-
-> Note: these requirements are slightly less onerous than those of Gitian builds
+- 16GB of free disk space on the partition that /gnu/store will reside in
+- 8GB of free disk space per platform triple you're planning on building (see
+  the `HOSTS` environment variable description)
 
 ## Setup