Add changelog to make preparation of a new release easier (#584)

It is encouraged that this file gets updated when there is a feature PR or important bug fix PR.

Author: howard0su

ChangeLog.md

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+**IMPORTANT:**\
+**You must install the filesystem along with the new firmware.  Make sure to save your models first!**\
+**The configuration of hardware mods has changed since the previous 4.0.1 version. Instead of the tx.ini, they are in a new hardware.ini file, just like they were in the last nightly builds.**
+
+
+## Installation
+
+    Follow the instructions in the User Manual that applies to your transmitter
+
+
+## Critical bug fixes
+  * Fix mixer ordering and mixer loop depdency.
+  * Significant reduce stick-to-air latency in FrskyX, Devo, and SFHSS protocols.
+
+
+## Major Features
+  * Support new transimitters:
+    - Deviation 12E, F4, F7, F12E,
+    - iRangeX IR8M
+    - Jumper T8SG, V2, V2 Plus
+    - RadioLink AT9
+  * Introduce a new format of localiztion file to save Flash space and speed up UI rendering.
+  * Scanner to check the RF enviroment around.
+  * Enhence telemetry support by adding some new fields.
+
+
+## Minor Features / Bug Fixes
+  * Reduce RAM requirement by optimizing stack and heap usage
+  * Reduce ROM size for AT9
+  * Support of Beken BK2421 / BK2423 support is removed.
+  * Fix USBHID under OSX
+
+
+## New protocols
+    * PXX protocol
+    * GD00X protocol
+    * Air Hogs "Star Trek" NCC-1701 protocol
+    * WFLY protocol
+    * Hitec protocol
+    * protocol for MJX Bugs 3, 6, 8
+    * SBUS, CRSF
+    * Flydream V3 protocol
+    * Corona protocol