Doc issue / make it very explicit that the mtar_t needs to be memset when using custom stream

[bsergean]

Maybe that should be bold in the README.md. It's my bad for not reading it ... but maybe that can save some time to someone else.

I spent a long time debugging something, I had tar->pos set to something garbage instead of 0 while trying to read from a custom stream.

But thanks for that library, it works great for us !