README.md: correct read size, explain EOF mechanism
README.md: correct read size, explain EOF mechanism

A read size of two bytes is a bit too short, this was a leftover from
writing and testing the script. Use a suitably large number instead.

Explain why EOF works like it does in smbclient_read, and that the
necessary strlen() runs in constant time.

aklomp/libsmbclient-php,aklomp/libsmbclient-php,aklomp/libsmbclient-php