add sx1262 example, update readme

[TristanWebber]

1. Created an example demonstrating how to manually add a board with SX126x transceiver. This also has the benefit of showing how to override methods in the HalConfiguration_t more generally.
2. Updated HOWTO-Manually-Configure.md to describe the HalConfiguration_t methods introduced for the SX126x transceivers
3. Updated README.md to introduce the new example

[terrillmoore]

Thanks! I'll try to review and merge tonight or tomorrow.

[terrillmoore]

I've reviewed. I'll merge it onto head, however please note:

1. the change log in README.md has not been updated
2. we've not updated the version number in the header file or in library.properties -- even though we'vd not changed anything in the core libarary, the arduino infrastructure basically requires a version bump. It can be a patch version, since core function is not changing.
3. we don't have any regression tests to check that the example compiles

I'll try to do this, but I guess I'll need to open a related ticket to track it.