move update instructions, add basic readme

README.md

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+#DF-structures
+
+DF-structures is the core information on which
+[DFHack](https://github.com/DFHack/dfhack) depends.
+
+For information on how to understand this format,
+see [./SYNTAX.rst](./SYNTAX.rst).  For how to update
+for a new version of DF, see
+[./how-to-update.rst](./how-to-update.rst)
+
+Originally dfhack used a file called ``Memory.xml``
+to describe data structures of the game. It explicitly
+listed addresses of known global variables, and offsets
+within structures to fields, not unlike the ini files
+used by Dwarf Therapist.
+
+However, as the number of known fields and objects grow,
+maintaining the explicit offset lists quickly becomes
+difficult, error prone, and inefficient.
+
+This format instead represents data structure layout
+purely via listing all fields in the correct order,
+exactly like a structure definition does in the C++
+language itself. There is still a file with numeric data,
+but it only contains absolute addresses of global objects.