Clarify effects of using php-http/discovery

[igorsantos07]

I got into several trouble trying to cleanup our project's dependency file, which is quite big. Turns out, after several hoops and loops, I understood that Mailgun forces a peer dependency for HTTP work - that's not clear since the dependency gets forced into our root require, Composer doesn't allow proper comments, and composer why can't help either.

I suggested them to output the plugin effects (php-http/discovery#258), but Mailgun's README could be clearer as well. Instead of just saying "we need some extra packages; if you're in a hurry, just install those two", explain that there's a plugin which will force installation of some PSR implementation, even if it's not one of those you're suggesting.