Add README.md

README.md

+# Build new image
+```
+cd /var/local/mkosi
+```
+If not done yet, install latest version of mkosi in a virtualenv
+```
+python3 -m venv .venv
+source .venv/bin/activate
+python3 -m pip install git+https://github.com/systemd/mkosi.git
+```
+Otherwise, just activate the virtualenv
+```
+source .venv/bin/activate
+```
+Build the image
+```
+mkosi -i -t directory -o /var/lib/machines/xcache-$(date +%Y-%m-%d-%H%M)
+```
+To start xcache with the new image create a symlink and restart
+```
+ln -s /var/lib/machines/<image-to-use> /var/lib/machines/xcache
+machinectl restart xcache
+```