doc/src/shares: update

doc/src/shares.md

@@ -1,17 +1,37 @@
 # Shares
 
+Persistent file-systems are provided by both volumes and
+shares. Volumes are block devices inside the virtual machine, yielding
+fast performance but mounted file-systems require exclusive
+access. Shares allow mounting an arbitrary directory tree from the
+host.
+
 In `microvm.shares` elements the `proto` field allows either of two
 values:
 
 - `9p` (default) is built into many hypervisors, allowing you to
   quickly share a directory tree
 
-- `virtiofs` requires a separate virtiofsd service which is only
-  started as a prerequisite when you start MicroVMs through a systemd
-  service that comes with the `microvm.nixosModules.host` module.
+- `virtiofs` requires a separate virtiofsd service which is started as
+  a prerequisite when you start MicroVMs through a systemd service
+  that comes with the `microvm.nixosModules.host` module.
+
+  If you want to run from the command-line, start `bin/virtiofsd-run`
+  separately.
 
   Expect `virtiofs` to yield better performance over `9p`.
 
+```nix
+microvm.shares = [ {
+  proto = "virtiofs";
+  tag = "home";
+  # Source path can be absolute or relative
+  # to /var/lib/microvms/$hostName
+  source = "home";
+  mountPoint = "/home";
+} ];
+```
+
 <div class="warning">
 When sharing a path that is on ZFS with virtiofs, the dataset must
 have options