Add jwdid example to the readme

[jonathandroth]

@mcaceresb when you have RA time (no rush, can be after the market), can you add an example of importing jwdid estimates to the README?

Blane Lewis kindly shared the following code with me for his example with the data here, although I think ideally we'd redo the example with the data used for the rest of the README.

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jwdid exp_personnel_share, ivar(district) time(year) gvar(first_elected_incumbent) cluster(district) never

estat event, post

honestdid, pre(1/3) post(5) vcov(e(V)) b(e(b)) mvec(0.0(0.1)1) delta(rm) coefplot `plotopts'

pretrends power 0.8, pre(1/3) post(5/9) b(e(b)) v(e(V))

pretrends, pre(1/3) post(5/9) b(e(b)) v(e(V)) slope(.663883)

return list

matlist r(results)