Update README.md

Update README.md

various fix, more user friendly warning

update doc to version 4

remove unused protobuf specification

max_traceback_factor -> traceback_threshold

add max_traceback_factor as a tuning parameter

limit traceback length in update_weight()

remove unused function

restrict max traceback length in update_value(), to limit the pull_worker response time

+time statistics for debug

change the type of PushData and PushLog from PUSH to REQ, as a fix for out-of-order problem

+floating point check for get_weight_sum() in get_data()

refine the push_data message to reduce retransmission

+check for reward_coeff