chore: prepare v0.3.6 release

[bdonlan]

0.3.6 (December 14, 2020)

Fixed

• rt: fix deadlock in shutdown (rt: fix deadlock in shutdown #3228)
• rt: fix panic in task abort when off rt (Support aborting a task remotely in a multithreaded runtime without panicking #3159)
• sync: make add_permits panic with usize::MAX >> 3 permits (sync: add_permits does not panic with usize::MAX >> 3 #3188)
• time: Fix race condition in timer drop (time: Fix race condition in timer drop #3229)
• watch: fix spurious wakeup (backport watch spurious wakeup fix for v0.3.x (#3234) #3244)

Added

• example: add back udp-codec example (example: add back udp-codec example #3205)
• net: add TcpStream::into_std (net: add a into_std method for tokio's TcpStream #3189)

[carllerche]

👍