small edits to first part of readme

Just a couple of edits I made while reading the paper. Feel free to reject some or all of them.

I Didn't know what you meant by "notches" in "Nodes add transactions to a new block and randomly try notches until a valid hash for a block is found." so I left that as is, but maybe it should be changed. Maybe you meant Nonces (I don't think truly correct since reusing them is fine)? Salts? Hashes?