IMPORTANT NOTE:
If the current firmware version of your NanoVNA-F V2 device is lower than v0.2.0, please upgrade to v0.2.0 first, and then upgrade to v0.2.1.

New features of firmware v0.2.1:
1. Display frequency range on save/recall slots;
2. Display Smith value as R+Xj by default;
3. Allow to disable marker dragging;
4. Automatically flip the marker when the trace is at the top of the screen;
5. Support segment sweep in nanovna-saver;
