Add some basic build instructions to run unit tests

README.md

@@ -44,11 +44,36 @@ This repository includes a test suite that covers most of the API and that is de
 
 Please help us improve the coverage of the test suite!
 
+#### To build and run unit tests
+
+The unit tests are automatically built by GitHub as part of pull request checks (in `.github/workflows/unit-tests.yml`).
+
+To build and run locally:
+
+**Dependencies**
+
+* [CMake](https://cmake.org/)
+* [GCC](https://gcc.gnu.org/)
+
+On (Ubuntu) Linux run:
+
+```bash
+sudo apt-get install build-essential cmake
+```
+
+From the project root:
+
+```bash
+cd test && mkdir build && cd build
+cmake ..
+make && bin/test-ArduinoCore-API
+```
+
 ### Implementing ArduinoCore-API
 
 In order to compile a core which is implementing ArduinoCore-API you'll need to copy/symlink the `api` directory to the target's `cores/arduino` directory as part of your development and release workflow. The most elegant and effective solution is to develop your core with `api` symlinked and produce the distributable archive by telling `tar` to follow symlinks. Example:
 
-```
+```bash
 tar --exclude='*.git*' -cjhvf $yourcore-$version.tar.bz2 $yourcore/
 ```