Add notes on variable names to README

[markolenik]

These expressions are valid in xpp (e.g. see Ermentrout 2001, p. 16):

dv.1/dt = -v.1
aux P.E.=21v.1

Functions, variables etc allow dots (not sure if prefix/suffix dots work though). To implement this feature you'd probably need to fix the regex in t_ID in the lexer and include dots, not sure if other things will then explode though.

[markolenik]

XPP also allows purely numeric variable names, which is crazy by today's standards. For example, the following works:

d0.11/dt=0

Since it's hopeless trying to convert such variable names to allowed Python variables, we'll explicit only allow variable names that do conform to Python's standard.