hosts: document the directory structure

hosts/README.md

@@ -0,0 +1,7 @@
+## directory structure
+- by-name/<hostname>: configuration which is evaluated _only_ for the given hostname
+- common/: configuration which applies to all hosts
+- modules/: nixpkgs-style modules which may be used by multiple hosts, but configured separately per host.
+    - ideally no module here has effect unless `enable`d
+        - however, `enable` may default to true
+        - and in practice some of these modules surely aren't fully "disableable"