Document keys in GlobalStorage

hacking/GlobalStorage.md

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+# GlobalStorage keys
+
+The GlobalStorage structure contains information which is shared among jobs.
+Jobs are free to define their own keys, but some keys have been standardized.
+Here they are:
+
+## bootLoader
+
+A dictionary with the following keys:
+
+- `installPath`: device where the boot loader should be installed ("/dev/sda", "/dev/sdb1"...)
+
+## partitions
+
+A list of dictionaries, one per partition. The dictionary contains the following keys:
+
+- `device`: path to the partition device
+- `fs`: the name of the file system
+- `mountPoint`: where the device should be mounted
+
+## rootMountPoint
+
+A string which contains the path where the root file system has been mounted.
+This key is set by the `mount` job.