Add a README for notes about the vvp target.

tgt-vvp/README.txt

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+
+THE VVP TARGET
+
+SYMBOL NAME CONVENTIONS
+
+There are some naming conventions that the vp target uses for
+generating symbol names.
+
+* wires and regs
+
+Nets and variables are named V_<full-name> where <full-name> is the
+full hierarchical name of the signal.
+
+* Logic devices
+
+Logic devices (and, or, buf, bufz, etc.) are named L_<full_name>. In
+this case the symbol is attached to a functor that is the output of
+the logic device.