Update README.md

README.md

+# Files/Directories
+
+- [/etc/systemd/nspawn/xcache.nspawn](etc/systemd/nspawn/xcache.nspawn): configuration for lauching containers from the xcache image, e.g. bind mounts
+- [/etc/pki/rpm-gpg](etc/pki/rpm-gpg), [/etc/yum.repos.d](etc/yum.repos.d): yum repositories and gpg keys on the host system - used by dnf to build new images
+- [/var/local/mkosi](var/local/mkosi): build dir to make new images with mkosi
+  - [mkosi.default](var/local/mkosi/mkosi.default): image definition file (distribution, packages)
+  - [mkosi.extra](var/local/mkosi/mkosi.extra): extra files/directories to be copied into the image (yum repos, [systemd service file](var/local/mkosi/mkosi.extra/etc/systemd/system/xcache.service))
+  - [mkosi.postinst](var/local/mkosi/mkosi.postinst): extra commands that are executed inside the container at the end (install extra packages not in the default repositories)
+
 # Build new image
 ```
 cd /var/local/mkosi