As far as I understand, you need it to to make an imperative читать - читай, писать - пиши.

I don't see any rule. Even Russian children can't get it intuitively and often say "писай", "писаю" instead of "пиши", "пишу" until get used to say "пиши", "пишу". So I am afraid that "-ать" verbs with imperative with ending "-ать" rather than suffix "-а-" and ending "-ть" are exceptions to be memorized.