[doc] add basic microvm documentation

doc/src/SUMMARY.md

@@ -14,3 +14,4 @@
     - [How-to]()
         - [Updates](./anleitung/updates.md)
         - [Rollbacks](./anleitung/rollback.md)
+        - [MicroVM](./anleitung/microvm.md)

doc/src/anleitung/microvm.md

@@ -0,0 +1,39 @@
+### Declaring a MicroVM
+
+The hosts nixosSystems modules should be declared using the ```makeMicroVM``` helper function.
+Use durruti as orientation:
+``` nix
+    modules = makeMicroVM "durruti" "10.0.0.5" [
+      ./durruti/configuration.nix
+    ];
+```
+
+"durruti" is the hostname.  
+"10.0.0.5" is the IP assigned to its tap interface.  
+
+### Testing MicroVMs locally
+MicroVMs can be built and run easily on your local host.
+For durruti this is done by:
+``` bash
+sudo nix run .\#nixosConfigurations.durruti.config.microvm.declaredRunner
+```
+
+It seems to be necessary to run this as root so that the according tap interface can be created.
+To be able to ping the VM or give Internet Access to the VM your host needs to be setup as described below.
+
+### Host Setup
+To provide network access to the VMs a bridge interface needs to be created on your host.
+For that:
+- Add the infrastructure flake as input to your hosts flake  
+- Add ```inputs.malobeo.nixosModules.malobeo``` to your hosts imports
+- enable the host bridge: ```services.malobeo.microvm.enableHostBridge = true;```
+
+If you want to provide Internet access to the VM it is necessary to create a nat.
+This could be done like this:
+``` nix
+networking.nat = {
+  enable = true;
+  internalInterfaces = [ "microvm" ];
+  externalInterface = "eth0"; #change to your interface name
+};
+```