Update README with release build instructions

README.md

@@ -30,6 +30,17 @@ nix build
 
 A statically compiled binary will be placed in the `result` directory.
 
+A full release, which cross-compiles binaries for all supported platforms, can
+be generated by doing:
+
+```
+nix build '.#release'
+```
+
+*NOTE* that cross-compiling will download a separate build environment for each
+target platform, which can take up quite a bit of disk space (>10GB).
+gigs).
+
 ## Configuration
 
 Domani is configured via a YAML file whose path is given on the command-line.