README: credentials no longer need to be passed in the URL per se

The better method is to pass them when the state resource is created;
update the README to reflect that.
smbclient_fstat: add function and documentation

This function reuses the code from smbclient_stat for a large part, so
this implementation is mostly a copy of existing work.
smbclient_lseek: add function and documentation
smbclient_ftruncate: add function and documentation
README.md: rewrite install instructions as Markdown

The lines

  ./configure
  make

were showing up as

  ./configure make

in the parsed version of the README.
Thread-local storage: add some documentation

Looked into thread-local storage when solving Issue 5 and learned a lot
about how PHP sets it up. Add comments to the existing scaffolding so
that it's easier to figure out if/when we need it later.