README: Update to point to target repositories.

As the yotta registry is being deprecated, this commit explicitly specifies
that the yotta target for MicroPython should be fetched from GitHub rather
than from the registry.

README.md

@@ -29,10 +29,11 @@ The move to the created directory and use yotta and the provided Makefile to bui
 You might need need an Arm Mbed account to complete some of the yotta commands,
 if so, you could be prompted to create one as a part of the process.
 
-- Use target bbc-microbit-classic-gcc-nosd:
+- Use target [bbc-microbit-classic-gcc-nosd](https://github.com/lancaster-university/yotta-target-bbc-microbit-classic-gcc-nosd)
+from GitHub (the yotta registry is deprecated as of 2021):
 
   ```
-  yotta target bbc-microbit-classic-gcc-nosd
+  yotta target bbc-microbit-classic-gcc-nosd@https://github.com/lancaster-university/yotta-target-bbc-microbit-classic-gcc-nosd
   ```
 
 - Run yotta update to fetch remote assets: