README could be clearer when working with bluetooth examples

[Riscster]

I have spent quite a long time trying to get the bluetooth-uart example to work and failing. Eventually (with a clue from micro:bit support) I realised the config.json was being ignored because I was copying it to the source directory instead of the root directory.

I think the paragraph in the README saying

n.b. Any samples using the low level RADIO APIs (such as simple-radio-rx and simple-radio-tx) require the bluetooth capabilities of the micro:bit to be disabled. To do this, simply copy the config.json file from the sample to the top level of your project. Don't forget to remove this file again later if you then want to use Bluetooth! For example:

should be rephrased to cover all the bluetooth and radio samples, specifically the bluetooth examples require the gatt_table_size to be changed to work properly.

Alternatively the examples could be restructured so they are

source/examples/<example name>/config.json 
source/examples/<example name>/source/main.cpp

so you could always do

cp -R source/examples/invaders/* .

to get both bits in the right place