truncated send always callback on writeable

Suggested by a Windows log where leaf.jpg meets EAGAIN and after
issuing the truncated send buffer, never sends any more

https://github.com/warmcat/libwebsockets/issues/111#issuecomment-39873129

Added note in README.coding about WRITEABLE callbacks able to
be generated by lws.

Signed-off-by: Andy Green <andy.green@linaro.org>