add a README

README.md

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+# Maxwell configuration
+
+The NixOS configuration of Maxwell
+
+## Switching configuration
+
+1. Mount remotely the secrets directory
+
+   `$ ssh -A maxwell sshfs wigfrid.ydns.eu:$PWD $PWD -o allow_other`
+
+2. Run nixos-rebuild
+
+   `$ nixos-rebuild test -I nixos-config=configuration.nix
+      --target-host maxwell --use-remote-sudo`
+
+3. Unmount the secrets directory
+
+   `ssh -A maxwell fusemount -u $PWD`
+
+## Testing changes
+
+1. Build a VM
+
+   `$ nixos-rebuild build-vm -I nixos-config=testing.nix`
+
+2. Change the open files limit
+
+   `$ ulimit -n 90000` 
+
+3. Run the VM without internet (to avoid a mess)
+
+   `$ unshare -nc result/bin/run-maxwell-vm`