add a README notice on how to work with promo codes

[serggl]

attempt to document some promo codes usage nuances (#156)

[serggl]

@SuperThomasLab what do you think about this one?

[Thomas-Vos]

@serggl Looks good!

One thing you might want to change is the following

One way to do this is to distribute your promo codes in form of a redeem link (https://play.google.com/redeem?code=YOURPROMOCODE) instead of just a YOURPROMOCODE values

to something like this. Just a redeem button that opens a dialog within the app, next to the purchase button. I think most users with a promo code will tap the redeem button instead of the purchase button.

BTW: It might look better if you add an enter in the README (line 205). But that's just my opinion.