As outlined via the change-log in the Mir Bazaar repository, 0.20 is quite a fine update. First up, Mir 0.20 brings support to allow screencasting to create a virtual output, which is needed for Ubuntu to support Miracast on Mir. Miracast remains the dominant protocol for connecting external monitors/TVs over WiFi. Android has supported this since the 4.x days and there's been various Linux desktop efforts like MiracleCast while it's great to see see Canonical/Ubuntu making more progress with their Miracast support for Ubuntu Phone/Tablet devices.
Mir 0.20 also has logging improvements, example improvements, various bug fixes, an option to use the Gold linker at build time, and many bug fixes. There's also a Mir bug that caused Ubuntu Phone not to be usable when a call comes in, which is now fixed with v0.20.