added changelog

changelog.md

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+# Adafruit nRF52 Bootloader Changelog
+
+## 0.2.7
+
+- Fixed PWM psel[1] is not reset
+- Fixed #41 move RXD, TXD into board header
+- Added Metro nRF52840 Rev A
+- Fixed #40 OTA issue with BLE_GAP_EVT_PHY_UPDATE_REQUEST e.g connecting with iPhone X  
+
+## 0.2.6
+
+- Fixed copy/restore current.uf2 issue
+- Fixed neopixel won't turn off after dfu cdc
+
+## 0.2.5
+
+- Make led pattern more consistent
+- Fixed issue nrf52840 not reset properly when upgrading bootloader+sd combo
+- Make led pattern more consistent
+
+## 0.2.3
+
+- Fully support Feather nRF52840
+- Update bootloader with new led pattern
+- Fix #203: return software timer handle
+