added changelog file

changelog.md

@@ -0,0 +1,8 @@
+# TinyUSB changelog
+
+## 0.5.0 (Initial Release) - 2019.07.10
+
+First release, device stack works great, host stack works but still need improvement. 
+- Special thanks to @adafruit team, especially @tannewt to help out immensely to rework device stack: simplify osal & control transfer, adding SAMD21/SAMD51 ports, writing porting docs, adding MIDI class support etc... 
+- Thanks to @cr1901 for adding STM32F4 port.
+- Thanks to @PTS93 and @todbot for HID raw API