Document how to add the required link to ArduinoCore-API.

Related to arduino/ArduinoCore-API#96.

README.md

@@ -30,6 +30,16 @@ to avoid duplicates.
 Contributions are always welcome. The preferred way to receive code contribution is by submitting a 
 Pull Request on github.
 
+## Developing
+
+1. Create an `<SKETCHBOOK>/hardware/arduino` folder, where `<SKETCHBOOK>` is the location of your
+  Arduino sketchbook.
+1. Change directories: `cd <SKETCHBOOK>/hardware/arduino`.
+1. Clone this repo: `git clone https://github.com/arduino/ArduinoCore-samd.git samd`.
+1. Change directories: `cd samd/cores/arduino`.
+1. Copy or symlink the `api` folder from the [ArduinoCore-API](https://github.com/arduino/ArduinoCore-API) repo (i.e. `ln -s <ARDUINO_CORE_API>/api .`, where `<ARDUINO_CORE_API>` is the location where you've cloned the ArduinoCore-API repository too).
+1. Restart the IDE.
+
 ## Hourly builds
 
 This repository is under a Continuous Integration system that every hour checks if there are updates and