Add transient-0 docs

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+# transient-0
+
+transient-0 is a binary encoding of hobgoblin data is for transient data where
+forwards and backwards compatibility is not required, such as IPC between
+different instances of the same program.
+
+The encoder and decoder must use the same version of hobgoblin and the same data
+definitions for decoding to succeed.
+
+transient-0 does not support:
+* Compatibility between different versions of hobgoblin. The exact encoding is
+  an implementation detail that may change between any two versions of
+  hobgoblin.
+* Compatibility between different data definitions. For instance, if a field is
+  added to a struct, there is no guarantee that data encoded before the field
+  was added can be decoded using the new data definition.
+
+